During my first pregnancy I wore my regular pants for a long time using the rubber band trick. Now I find myself longing for the comfort of the full belly panel even before I'm truly showing. There are lots of cute maternity jeans/pants out there... even designer versions, but I also noticed a few DIY tutorials and thought I'd give it a try.
I had these white skinny jeans that had been really inexpensive but I hardly ever wore because I didn't really like the back pockets. They have a bit of stretch to them, so I thought they'd work well for this alteration. My favorite tutorial is on Grosgrain, here. She gives really great step-by-step instructions with pictures.
The one thing I did differently is that instead of making my own panel, I used a "Belly Band" that I already owned. I like the thinness, the stretch and smoothing support of this over a knit folded in half... and the panel doesn't slip down like a knit might. It also makes this alteration even easier. My favorite belly panels are from Forever21.com. Sadly they don't have them on the site as I'm posting this. They were only $7.80 each and are the most supportive, longest (covers the entire belly, not just up to the belly button leaving a line at your widest point) and definitely the most affordable ones I've found.
I think the most important step is to try on the band and the pants together before sewing, as Kathleen mentions, and make some marks where you will pin the two pieces together. You will want more of the panel in front than in back, as opposed to just attaching the band evenly. I was prepared to trash these pants if it didn't work out and was happy that it went together smoothly.
I recently saw a love/hate conversation about skinny jeans on Facebook and I had to laugh because those opposed to skinny jeans in general would certainly be appalled by my pregnant channeling of Tweedle-Dee or Tweedle-Dum here. I say to each his own. Whatever style you like, give your pregnant self the gift of being able to breathe and move without constantly hitching up your pants!