If you don't have a personal trainer watching over you, how do you ensure you don't get injured when you're strength training? My biggest fear is getting injured from something I can't recover from.

So I like to work out from home, in the past I had luck following videos like P90X, so I'll use that as an example.

If I use P90X for their strength training routine (3x per week), and I do the modified exercises (as countless blogs have talked about and mentioned), how do I ensure the exercises I am doing are right without a personal trainer?

Do you guys use a mirror? Video? Do you record every session and gauge it afterwards?