first the more you prune the great the future harvest you lose.
second try supporting you plants using a good tomato cage works better then staking.

exmar - I suggest some reading over on the Growing Tomatoes forum here (linked at the top of this forum's page) for more info on the pros and cons of pruning tomato plants and for many discussions on proper tomato plant supports.

Good supports and proper spacing of plants render picking problems and pruning, which most experienced growers shun, a moot issue. If your plants are staked and you still can't reach in to pick fruit then I suspect your plants are heavily over-crowded.

As lacysue said, pruning reduces production. The more you prune, the more production you lose.

Dave

Hmmm, My plants are 4' apart in rows that are 5' apart. So my plants aren't "heavily over crowded." The issue is that the San Marzanos develop a lot of stems from the main one and tying loosely to the stake results in a interwined tangle. San Marzanos can easily grow to 10 feet in length.
The tomato cages that I see at garden centers are about 4' high with 6+ inches to be inserted in the ground. THAT is the reason I use 2"x2" stakes 6' long driven a foot in the ground.
Granted, pruing does decrease production, however proper pruning has been around as long as agriculture. e.g. If you don't prune fruit trees you'll end up with a lot of little fruit instead of the size that you want. Also, ever not prune your grapes??? :-) Also, I believe that tomatoes are fruit?
Guess I'll do some judicious pruning and report back if anyone's interested.
Ev

You'll have to make your cages. Use a wire with a large square that is easy to reach through.

If you use the right diameter, you can reach in and reach the middle. The plants will have more room, so branches shouldn't be so crowded that you can't reach in there.

I'm going to guess that for that variety of tomato, you will want a fairly large diameter cage, but not so large that you can't easily reach to the very center of it.

I prune all of my tomatoes and that works fine for me. I've done that as long as I've gardened - many years now. You can use a cage, but then it's harder to reach into the thicket and find the fruit. I prefer to trade lots of fruit for ease of picking. I still get plenty for sauces, freezing, and eating fresh.

On the other hand some people don't stake at all, but let their tomatoes sprawl on the ground. I'd lose a lot to turtles if I did that, and it also requires a lot of space.

It's really a question of personal preference. All the methods have advantages and disadvantages.

Buy yourself a roll of concrete reinforcement mesh at your local home improvement box store. It comes in rolls 150 ft long and 5 ft tall, with a 6 inch grid. Make your own cages by unrolling the wire and cutting them to the diameter you wish. For your tomatoes ( have some San Marzanos as well, so I know what you mean) I would make the cages about 30 inches in diameter, which would require 94 inches in length from the roll of wire, or a little less than 8 ft. For $100 you can make yourself 19 cages that are far superior to what they sell at most stores.

Thanks for all the good info. I kept hearing about cages, but the ones I see for sale just couldn't work. Next season I'll look into buying some concrete wire and make my own.