When it comes to precum, some people have it, some have lots, many don't ever at all. I'm one of those people. I think, like maybe twice have I ever had a drop of little clear precum fluid. From what I understand though, it has a lot to do with how long you're aroused before you ejaculate. When you're at home masturbating it's most likely that you pretty much get to business, but when you're with your GF you're probably aroused for a much greater period of time beforehand, anticipating whats going to happen, and messing around.
As for ejaculation force, I have a few tips. Excercise your PC muscle. This is the muscle used to shoot your semen when you cum. It's also the same muscle as when you cut off the flow of urine when urinating. AND, the same muscle you use to clench your ass shut. You can actually excercise this muscle while sitting on the computer, watching TV, in the car or at school. Anytime you're sitting down. Just clench the muscle, hold it for like 2 seconds, then slowly release...and repeat. Do this as many times as you can, or have time for. Do this regularly and i GUARANTEE stonger, more powerful orgasms. You can actually get a bit of pleasure off of this too, because flexing that muscle stimulates your male prostate gland. Also do this while masturbating.
Additionally, drink lots of fluids, cause more semen will have more tendency to shoot more. AND when masturbating, the most important thing is don't ever worry about the mess keep pumping it hard until every drop is out. I think a lot of people kinda stop jerking so fast, and kinda stop or go really easy when they're about to cum. This also makes for a less explosive orgasm.
Hope I could help!