The trick isn't to buy a ton of turrets and spread them out.
For the first dozen or two waves, you should only have 2 turrets, all maxed out.
From then on, you should be focusing on building 1 turret at a time, and maxing it out.
Also, spreading them out is dumb. Putting 3 on corners is great, since they shoot fastest at 90 and 45 degree angles, and a large accumulation of them in a single spot will wear out the enemy faster for when they hit the next set of turrets.
If you can get 2 or 3 turrets on each corner, with a good deal of upgrades on each, you will get a long way without having to do much.
Also, start out with only 1 water tower. They are good because they have a better damage increase for each upgrade. However, don't waste the extra $1000 on the last upgrade. Instead, keep that money for all of the seed-towers, as the jump to it's final stage is something like 9,000 damage.
Use the water tower to get the money for the first few seed towers, then buy and upgrade nothing but seed towers afterwards.