I ride alone a lot. I stick to populated bike paths or known routes where DH would know to come look for me. I leave a note on the counter with departure time and anticipated arrival time (when I was single, I used to call my answering machine with the info!)

Like everyone said above, carry everything you would need. Even if you aren't comfortable changing a tire, don't expect a friendly cyclist to have your tube with them (air cartridges are much easier on the side of the trail).

I also ride with a DH who rides so far ahead, I might as well be alone. He can't hear me yell, and sometimes, doesn't answer his phone! He'll eventually turn around, but not before I could change it myself!