Random encounter tables using two dice are heavily peaked toward the median. Sometimes that's good. For instance if you want the weather in a region to be fairly predictable are trend strongly toward a specific result. But sometimes that median result becomes tedious when rolled repeatedly. This is especially true in encounter tables, where the "most boring result" is often stored in the middle. After you have fought that random, roving band of goblin guards three times, do you really want to chance rolling it again?
There are some cheap, fast, and easy fixes.
1. Cross out the result you are tired of getting and write in a new result. Goodby "d4+2 Goblin Guards," hello "Bugbear looking for the latrine."
2. Just ignore the dice when you get a result you have already gotten and choose a different result. I recommend shifting up the table one result at a time until you hit something new.
3. Instead of using the result you get (yet another run-in with those gobba-guards), relate a sign from the most dangerous thing on the table. The party turns the corner to see a huge pile of dragon poo!
4. Swap out the two like dice for two unlike dice. For instance, change 2d6 into a d8+d4! This flattens out the curve, as seen in these graphs (courtesy of AnyDice.com). Note that the chances of the extreme results don't change that much, but the middle flattens out dramatically.