I use a Tony Pena knot which casts beautifully through the guides:

You can read about how to do one here:
http://www.frenzytackle.com/tips/pena.htm
I've tried everything but have settled on that knot.
Great very easy to tie knot especially good for joining 30 to 50 pound mono to 40 to eighty pound spectra.
I like it cause I can tie it fast and get it to come out right every time. That said if you don't get the mono knot tight it can slip, but it's better then a albright or uni to uni in my opinion.
A trick I use is: after the knot is tied I just touch the mono knot end with super glue, and since I have started doing that I have never had it slip or fail in the field. I actually carry super glue in my gear just for topshots.
All that said: The Bob sands knot is the same not with a uni on the mono end instead of a overhand knot. It's supposedly stronger but it's harder for me to tie, and it does not cast as well as the Tony Pena knot.
Good luck, Jim