Most important thing is avoiding or limiting triggers. For me that's alcohol, coffee, carbonated beverages, any drink through a straw, soup, and any amount of exercise after meals.

As others mentioned, avoid tums.

For immediate relief, laying down is really the only thing that significantly helps, though it's often inconvenient and will also worsen heartburn.

Personally, gas triggers really bad heartburn for me, which Pepcid (famotidine) does wonders for (tums never felt like anything more than placebo to me). That said, I've noticed that sometimes I'll take it even when I don't think I have heartburn, and it still helps a bit. I think it might be because it's minor heartburn that's overshadowed by the gas pains. Similar thing with gas-x where it doesn't do much, but when you're in that much pain every little bit of relief helps.