Of more value is your resting Heart rate before you get out of bed in the morning. This will tell you the state of your health.
Ramboscie

This worked for me. Better than any planned work out you could possibly do.

1. Do a surfing run on a big, "honking" day.
2. Three fourths into the run, which for me was about 8 miles into it, wander a little too close to a break that is roughly about 2 miles off shore and usually doesn't break unless of course it is a big "honking" day.
3. Paddle your ass off to miss the freak sets, which were probably about 15 feet Hawaiian.
4. Finish the run then look at the max heart rate of that run.

Guaranteed!!! You'll reach a heart rate that you could otherwise never reach by doing anything suggested above!!!!

effectivepull - that's what I was trying to say, too.
Your max heart rate just shows that your young, healthy, old, sick etc. - as Rambo says
You can use it to calculate 'zones' in which you want to keep your HR while training.
More valuable/precise is your lactate threshold; that is the level of exercise /~ HR when your lactate levels suddenly start jumping up quickly.
The target is to move that threshold so that you can train/perform longer without that lactate jump.
I think Rambo said all that above already.
The '220 - age' was the first useful formula that two guys established. It is still used in the protocol when you do a treadmill. It is just a rough guide, doesn't mean much.
Karel Jr monitors his lactate levels.
For those not that familiar with it, simplified version:
Lactate builds up due to muscle break down during exercise. To a degree it can be cleared. If the exercise intensity gets up too high, it will build up in the body and interfere with many functions.
That happens at the lactate threshold.

eck, max HR is max HR. Think you're talking about anaerobic thresh hold. Rambo's method will work, a good warmup, a series of increasingly difficult intervals culminating in a final blow chunks one. Fun or what?

I believe he magic number is not max HR, but your HR at your lactic acid threshold.

Why not just spend the bucks, and have it determined professionally?
So go see your doctor and if they don't do it, have them write you an order to have it done. Make sure they specify the "super max protocol" for trained athletes, and not the every day common garden variety they give out of shape patients. There is a difference!!!! After all, you want it taken to the max!!! You can also bet it will show any hidden heart ailments too! Worth it.

You can also train the body to "tolerate" high lactate levels, you can also take a supplement to neutralize or buffer lactic acid.
You can also get faster by sitting on the couch with a beer and watching Jr surf the ocean.
Yours to choose.
Rambo

Now, any tips on how to keep the damn thing from sliding down? Seems like the natural movement of paddling tends to push it down below your lats.

You could use some spray glue like what swimsuit models use to keep the bikini bottom from riding up their backside while walking or their tops from sliding off while moving.
Or you could try to make some type of "suspenders" to keep it up where it needs to be.

Actually 220-age (Karvonian formula) is extremely accurate for many. The issue with HR training is everybody's different. Karvonian method is result of developing a universal formula for the majority when all the data resulted in a bell-curve. Of course the problem is all the folks on either side of the median. You could get a test at a facility to find max HR, VO2 Max, Lactate Threshold, etc... But, your numbers will change with training, fitness levels, and age. Once you find max HR, as far as where to train depends on current fitness levels and what you want out of it. There are zones (say 1-5) to train in depending on what you're trying to achieve: base training, recovery, sprinting, etc...
The strap will be lower than you think (below xiphoid process) but will still transmit just fine. When you put it on ensure good connection with spit or the jelly they sell for it. Spit works fine for me then sweat keeps conduction decent.

For the sake of discussion: none of these formulas is extremely accurate.
The Karvonan formula is accurately describing the statistical median of a population.
You have to factor in what exercise was used for the Karvonen model, too.
Most equations offer roughly the same degree of accuracy.
The equation “220 - age” is more convenient and less complicated than others.
Max HR in swimming is different from max HR in cycling; statistically about 6 beats lower if I remember correctly.

pk and joe, not sure if either of those will make for max HR or just the need to get a new pair of shorts.

"I know that there is no way that I can reach my cycling max HR in the canoe."
I have the same problem with running and paddling. I have a good gauge of perceived effort with heart rate, but with paddling it seems that i'm working a lot harder at the same heart rate. This maybe the result of significant ineffeciency in paddling. Time to paddle some more.

Get on an indoor training bike, place some matresses around on the floor, wear a helmet, warm up, cycle fast for 5 min, then faster for another 5 min, then faster for ...
Just before you loose conscience and fall on the ground, note your HR : this is your max.
Now if you're still alive, go and get a garmin.

jp,
HRM's with strap pick up electrical signals from nodes in the heart (not sure which ones), wrist-style HRM's measure just pulse rate I believe. Therefore I would expect more accurate results from chest strap transmitters.
After much research, I settled on the Garmin Forerunner 50. I wanted just a plain old HRM that had direct download capabilities to PC. Polar pissed me off with all their bells and whistles plus unless you are willing to spend over 200 bucks you have to use thier online web interface for downloading which is useless. Garmin hit the money with the 50; under 100 bucks, straight-up HRM with download capabilities, and and you can swim wiith it. I end up using it as my main watch too, but it uses up batteries every few months thought thier cheap. HRM market didn't have any competitors, although it might now.

@Chineboy - I read that same book. It's phenomenal. I learned that I have been taking it to easy on my runs for quite some time... on second thought, maybe it wasn't such a good idea to read it. ;)