GMKlenklen,

In the case of 90 degree corners, I think artdutra04's answer is the best given the stock connectors. The triangular uppler/lower corner brackets (with 3 bolt holes on each leg) is the most secure option for bolting the two pieces together. Assuming the main forces will be pushing the two toward each other (i.e. if this is for your robot frame), then the second best option is using the inside corner bracket. If you need to put something close to the inside of this angle, then the outside corner bracket and an angle bracket may be the way to go.
Pillow blocks can either be bolted to a biscut in the frame, or if you're willing to give up the adjustability of the pillow blocks, they can be bolted through the frame.
In my expereiences with my former team, I think we may have used all of the methods above at some point or another, and our robot never fell apart (although we normally find a bolt or two lying at the bottom of the shipping crate each time we move it...)
My apologies if I seemed a little curt; I had just finished my statics homework, so I was still in that nit-picking state of mind. Best of luck this season,

Mike