When to start getting aggressive

[TEBPilot]

I mainly play the $20 SNGs at Party poker and find myself in this situation very often. 5 or 6 players remaining with my stack somewhere around 500-600 at the 50/100 level. Most of the time this is a result of playing tight but not getting any cards or having a good starting hand and not hitting the flop. At this point, raising 3xBB doesn't seem to be an option as I am dumping 1/2 of my stack or more so I usually just push.
For you players who are consistently winning at this level, when do you start getting aggressive in this situation? Do you limp into some pots and try to steal after the flop when rags hit? Should I be more aggressive at the 25/50 level? Should I just be patient and wait for any pair/A10 or better to push or is pushing with any two the proper move?
I don't feel like I'm playing poorly but my ROI after 400 games is a whopping 10% so maybe I am playing poorly [img]/images/graemlins/mad.gif[/img]

Thanks in advance.