Pea straw is the way to go
(Melb, Victoria, Australia)
I like to use pea straw about 2 inches thick to mulch tomatoes.
I find when I water my vegetables I can use the "jet" setting on the water nozzle and squirt at full blast, which really penetrates the straw without throwing up mud.
I am continually surprised at how the moisture is retained. On a hot summer's day, I'll stick my finger through the straw into the ground below, expecting it to be hard and dry, only to be pleasantly surprised at how cool and moist the soil is.
The one drawback with pea straw is that it sprouts pea shoots. It's not a such a great problem though, as they are easily pulled out. Plus, being rich in nitrogen, the pulled shoots left in your tomato patch will break down and add to the fertility of your garden.
At the end of the growing season, just turn the soil over. The pea straw will mulch during the off-season. You know you're doing the best thing possible for the soil in your garden by adding organic matter to the soil.