1.Look-ahead and this is the most important thing. For megaminx it's a lot harder to tracking pieces.You need lot of practice. I don't see any more things you can do.
Algs for LL are not needed 2 look LL is enough.
When I recently got my megaminx I was around 4:30. Now, after maybe 30 or so solves, I'm scraping 3 minutes. Just keep at it, look ahead and solve the star (cross) on the same side every time (If you're not already).