If you like the multi-blade razors, I used a Fusion for a while (via a free sample). What I liked about it is that on the edge opposite the four blades, there is a single razor, for edgeing. You may find that single bladed edge better for keeping the sideburns. I wish they made just a single blade razor like that - with just the blade on the edge (instead of the typical thick-headed safety design).

Wish I could help more with how not to make the edge creep up though. I know when I kept my sideburns down to the bottom of my ear, the same thing would happen to me. It always reminded me of this Blondie comic strip I saw around the time I started shaving where Dagwood is shaving, trying to get his sideburns even, but has to keep touching them up, then in the last panel, he's eating breakfast with a mohawk.