How do you chop the veg? If you use a blender, stop it this instant!
No, seriously, it sounds like you might be over-processing everything -- pureeing instead of chopping, and beating in too much air in the process. If you don't want to chop everthing by hand (best for texture), at least do each veg separately and then mix them all together. That way the softer ones won't puree while the harder ones are still chunking away.
Shroom's ingredient suggestions are good, although I like a LOT of cilantro. I also like to add some diced jicama, for extra crunch and sweetness, and/or red or green bell pepper for the same reasons.
Actually, you can make salsas from all sorts of veg and fruit. If you're interested, I'll post some ideas (and others probably will, too).