The biggest trick I've found is using garlic effectively. Funnel all the zombies into fewer rows and your economy works much better, as you can narrow five lanes down to two, or six into three. Make the garlic 2-3 deep for easy management, and you can plant a bunch of sunflowers safely behind your garlic rows. Combine that with watermelons (even better with the freeze upgrade) and you're doing a lot of damage to multiple zombies with each shot.
The sequel will almost certainly be a Steam pre-order, as the brilliant minds at PopCap rarely disappoint, and PvZ has that awesome balance of casual accessibility and pretty hardcore depth and strategy.