Make sure the cabinets are SQUARE to each other, LEVEL, and snugly connected. That ensures that the countertop will drop into place with no hassles. Do a dry fit to make sure.

Then all you need to do is apply caulk to the joint. I used a flexible unsanded caulk for tiles because it came in a color very close to the countertop's color.

Another counter got clear matte silicon caulk because it abutted a different material.

If I were doing it, I would use biscuits or dowels, and yellow glue to assure a tight flush joint stays that way.

We used something like this, but we metered our corners, so I'm not sure if that makes a difference. http://www.woodcraft.com/product/27C02/joint-connector-small-212-to-314.aspx

The walls dictate cabinet and countertop placement mostly, so if they're out of square it's no big deal you just have to properly bisect the angle if you're going to miter.

I'm not wild about silicone for seams in countertops, but that may be because I've fixed so many failed ones. I'd go with some Titebond 2 and the draw-tights linked above.

thanks everyone! Thankfully the person that installed my kitchen's cabinets in the 90s installed them perfectly level and they are nice a flush to the walls :). For the joints I went with dowels and adhesive.