Glue

Do be careful with Cyano glues like Zap A Gap though. First of they come in different thicknesses, the best all around would be the medium thickness glue. It also is great for filling in small gaps in the plastic. Second is that if you are not careful with this glue you may become awefully attached to whatever kit you're working on.

I use "Zap A Gap" for bonding metal or resin parts to themselves or styrene. I like it cause it holds tight, but gives you a couple of minutes to adjust location if need be. (In my case it's always needed)

For photo etched: Cyano acrylate.. Same goes for resin. Styrene usually does best with liquid cement.