So how do you get better rolls? If you're going to be all scientific about it, you don't. Blizzard has gone to all kinds of trouble to make sure that random rolls are as random as they come. They've even put measures in the game to make sure players can't spoof rolls-- roll "emotes" are yellow, not orange, and rolls are generated by the server, not the client software.
But if you want to be silly about it, there are a few methods. Some players swear by keeping Lucky Charms in your bag (while it would be awesome if that worked, it doesn't). Others say higher intelligence helps rolls, and have special gear just to use for good rolling (again, that won't work either, but it makes me laugh). And still other players believe the /rolls are set to a certain time-- they'll roll on the hour or minute just to try and catch the server on a particularly high random seed. Doesn't work either, but don't tell that to the players who swear by it.
Here's what I do: whenever I'm on a particularly bad streak of rolls, I warn the group that I'm "clearing out my bad rolls," and then run /roll a few times. You'll get a few low numbers (34, 20, 52, 30), and then you'll get a higher number (something like 84 or 91). At that point, you stop rolling, and then the next roll, you'll win the loot roll. Again, according to science, that's not supposed to work (and it probably doesn't, except in my head). But believe me-- my characters have become better geared more than a few times because I "cleared out the bad rolls."