Is saving 10% of your paycheck every month enough? 20%? 50%?

How much from every paycheck should you set aside for yourself? How much should you pay yourself before you pay out to anyone else?

If you set a plan for how much of your income to put away each month, and then do some math, you can project how much you'll have so many years down the road. The trick is in choosing what effective interest rate you'll get in those so many years.

But as the fine print always says:

past performance is no guarantee of future results

You can never know what interest rate you'll get on your money in the future. The corollary to this rule is, you can pick whatever rate you want to use. It doesn't matter, because it'll be wrong, but it will be as right as the next person's guess. That means whatever interest rate you use, you will be right, or as right as the next person's at least.

When is the best time to invest?

Is now a good time to invest?

Should I be in the market now?

I get these questions, and variations on them, all the time. I get them from friends, family, even from people I've just met at a party or other gathering. I'm not a financial planner or a broker or any kind of fiduciary for that matter. People ask me this because they know I'm in the market. They think I know what I'm doing in the market. They think I'm some kind of expert. Somewhere I got the reputation for being "in the know".

Here's what I tell them: I'm no expert. I don't know anything more about where the market is going tomorrow than the next guy. Then I tell them I'm invested. I tell them the best time to invest is right now. I tell them that if they're not in the market now, they should be.

You know why? Because the average investor won't have enough money to retire on and live happily ever after if he doesn't invest what he's got. It doesn't matter if you're 24 or 42 or whatever your age, sitting on cash and waiting for the right time to invest is a losing proposition.

Welcome to Friday. I don't know about you, but this week has gone by relatively quickly for me. I've had more than my fair share of challenges at work this week, and the weekends bookending these workdays are packed solid. There's nothing like a full calendar to make the time fly.

And of course, watching my portfolios appreciate tens of thousands of dollars in mere hours doesn't hurt at all.

To celebrate Friday, this old dog will be taking the afternoon off. I'm headed for a retirement planning seminar over lunch and into the early afternoon. After that it'll be time to hit the links for a round of golf.

I'll report back next week sometime on what I learn at the seminar. I'll tell you this right now. My first reaction on receiving the invitation was that I didn't think I'd have anything to learn. In fact I would have tossed it straight away had the invite not come from an old friend now in the financial planning business.

But I came to my senses and realized that if my impulse was to toss the invite without thought, I was probably a bit overconfident. I probably have something to learn. I'll find out later today if there are any new tricks this old dog needs to learn.

I'm always preaching investment advice to people I know. Invariably I'll be having lunch with someone and even before the food shows up I'm going on about how to avoid making one or more investment mistakes.

One of my biggest pet peeves is holding on to a loser investment too long. Time and again I hear the sob stories about the stock that went up briefly and then tanked. The so-called investor, a.k.a. deer in the headlights, then freezes, hoping the stock will rebound. Too often it continues to fall, or does nothing at all, and our investor friend continues to lose money. These are the kinds of things that cause us to under perform the market.

But today my story is about another kind of under performance.

Our city, like many others, struggles with growth in population. Economic prosperity is a double edged sword. The good is easy to see and well worth the costs. But the costs can be significant and painful to bear. Our city and the surrounding area are struggling to keep up with rapid expansion in population. We're faced with having to not only build new infrastructure to accommodate geographic expansion but also wit the rebuilding of old infrastructure. Old roads and highways which were built to convey a much smaller population from point A to point B can't contain the crush of traffic demanded today.

I think people aren't so inconvenienced by the building of new infrastructure. Every new lane change and detour or delay is only temporary. It's just another step on the way to a fresh new highway which will soon cut commute times significantly. Replacing old roads and highways is a lot harder to deal with. Tearing out old roads greatly reduces capacity in the near term. The pain of renewal is much more acute than that of new building.

Today should be interesting. Yesterday's news said August foreclosures doubled. More than 100,000 homeowners received notices of default last month. Almost 2/3 of them were from California (40,000+) and Florida (26,000). Sound serious? You bet it is. Now the fears are real that our little sub-prime mortgage problem will lead to a recession. From the news I've been reading, there is little doubt the Fed will drop interest rates today.

The decline in housing is now in its second year. With no help having come so far from the Fed, homeowners with adjustable rates have been stuck, frozen in place as their rates adjusted and put monthly paymens out of reach. The rate cut might help if homeowners in over their heads can refinance at more affordable rates.

The problem many will face is that the value of the homes has fallen. That fact may make it difficult for banks to extend loans for the full amount of the original mortgage amount. Remember, the new mantra in real estate lending is caution and conservatism.

This weekend I was talking to one of the new neighbors who had just moved in a week or two ago. We chatted about the great fall-like weather we were having and how great its timing was for his moving in - mostly small talk, getting to know you kind of stuff.

As we were talking, his son walked up to the conversation. His dad introduced him. He said hello, and in the single-minded way only kids can truly master, asked his dad if he could borrow $50 for a new skateboard.

Michael's dad replied with some of the most sage debt management advice I've heard:

Who knew there would be a sliver lining to the dark cloud of rising gas prices? Charles Courtemanchey of Washington University in St. Louis thinks there may be a link between the price of gas and the size of Americans' waistlines (PDF file link). He wants to prove that higher gas prices will mean less obesity over time.

A causal relationship between gasoline prices and obesity is possible through mechanisms of increased exercise and decreased eating in restaurants. I use a fixed effects model to explore whether this theory has empirical support, finding that an additional $1 in real gasoline prices would reduce obesity in the U.S. by 15% after five years, and that 13% of the rise in obesity between 1979 and 2004 can be attributed to falling real gas prices during this period.

It's funny sometimes how you can watch these little wavelets go around the financial community. It's sort of like the waves that get started at sporting events except in this case it's writers picking up on each others' stories and taking a topic a little further, or off in another direction.

I recently saw a little wave of articles asking and answering the question about what to do with extra money:

Given some amount of extra money, should I invest that money, or would it be better to pay down debt?

I just couldn't jump on this one because to me it always seemed so obvious. The best strategy is the one that leaves you with more money in the end. The problem is only a little complicated because all debts are not treat equally, just like not all income is treated equally, in the eyes of the IRS. The answer relies on how the interest paid and the income earned are treated when it comes to taxes.

A while back fool.com put up another one of their sales pitches entitled "Don't try to beat the market". I checked it out. With a title like that how could I not?

What you get, in a nutshell, is a come-on for Fool's Hidden Gems newsletter. The pitch goes something like this:

1. You can beat your brains out and use up all your time picking stocks. It's a brutally difficult way to beat the market or...
2. You can buy an index fund and get the same returns as the market or...
3. You can go the smart route and let someone else recommend which stocks for you to buy

The Fool's Hidden Gems is their small-cap value stocks newsletter service. If you follow the link at the end of the "article" you'll find a very long sales pitch for Hidden Gems. You'll read several dreamy stories about fabulous returns, whetting your appetite for your first 100-bagger.