I use Loctite Control Gel Super Glue. It works great on PLA.
Here's an example of a design I did...
And it glued together with Super Glue
It's almost impossible to separate the glued parts. The print will break before the glued area does. If I've been handling the part a lot, I wipe it down with rubbing alcohol (same goes for painting a part) to get oil off it.