I would emphasize that United is an "elite-heavy" airline, so it's tough to be upgraded even at a level such as Gold. But that all depends where you're flying, what type of aircraft, etc.
One more piece of advice that I cannot recommend enough: join the Global Entry program. This will allow you to go through the TSA PreCheck line. I fly regularly on United out of O'Hare and this has saved me plenty of time and hassle. You have a dedicated security lane that rarely has a line, and you don't need to take off your shoes, belt, jacket or anything (e.g. laptop) out of your bag. For someone that travels quite a bit, Global Entry is probably the best $100 I've ever spent.
I went through the application process several months ago, and have gotten accepted in the PreCheck line every time since (you have to scan your boarding pass and hope for 3 beeps).
Edit: It's also nice to have the option of US Airways if you choose to fly United and vice versa. I was booked via US Air and had a two hour delay. I walked up to the counter and ask if they could transfer my ticket over to United since their flights were running on time. They transferred me and I was home right on time instead of hours later. Not sure if that's typical procedure, but it's nice to have the option.
