One thing to keep in mind is the substance you're using. Some substances clog easier with the smaller gauges. I hate to say it but you may find that if you have smaller veins, or are only able to hit those veins, and the substance is apparently more thick, you may be out of luck.
I would consider though what the above poster above said. 28 gauge is the usual for almost anyone or anything when it comes to street drugs - I personally usually go for about 29, and would rather take 30 than 28. However, depending on the day or the drug, 28 is suitable.
I hope this helps.
-dp