To help w/o changing the soil you can:

* Drill extra holes in the bottom of the containers and then elevate the containers on bricks or boards. This isn't to increase drainage, but to increase aeration of any roots that get to the bottom of the pot. Should help out tomatoes a lot. This has worked well for me.

* Haven't tried this yet but will this year: Just poke a long dowel into the mix and remove. This will leave an "aeration column". Poke several of these in each pot, similar to how they aerate grass lawns.

* This one worked well for me last year. In a tub with heavy mix I dug out a column of the mix from top to bottom and put in wood chips. This also formed an "aeration column". My pepper plants in this tub did much better after I did this.

Here is a link that might be useful: Organic container growing

Thanks, how big should the holes be for the 2nd method?