Does anyone know a good DIY pretreatment for carpet?

It has been my experience that most home brew cleaners may work well but on tough projects they don't hold a candle to top rated commercial cleaners. Since you have a project that you believe will be insurmountable, why not give it the best chance of success? I cannot recommend a specific brand but there are a whole gaggle of spot treatment chemical magic in a can products out there. Pick a brand name you know and trust.

If it's a light-colored carpet, use hydrogen peroxide. You will be amazed.

Oxyclean and simple green will get even blood stains out.

Thanks everyone! I'll be giving these all a try!!!

I have tried so many things to clean my carpets even with a steam cleaner. I have stumbled on the BEST way to pre-treat them. I use an oxygenated cleaner (my favorite is Awesome Orange that I got from Amazon). I mix Awesome Orange in a gallon of water (2 scoops) and dunk a towel in it and soak the carpet and scrub it. Rinse and you will see how much dirt comes up this way. Then use the same mixture in the steam cleaner instead of the commercial shampoo. It works so well!

It really works....I have 3 cats and a dog and my carpets look good!

Resolve carpet liquid cleaner, the kind you use with a shampooer. Use full strength, I manage to get grease out of the carpet, when you come room the garage, to the house.