A technique I used was called "20-rep squats." It's been written about in a few places. Simply put, you choose a weight that you can do 20 squats with using good form. Next time you add weight to the bar and do 20 again, lather, rinse, repeat every week or whatever your interval is.

It works well for working out alone because the goal is simple: do 20 reps with more weight than last time, even if it's just 1 lb more. You simply don't allow yourself to stop before failure. It's "20 or die." It was the most productive form of weightlifting I've ever done in terms of increasing strength and body mass.

Make sure you have a good, stable squat rack :-)