(As with everything) I didn't invent this, but I use the heck out of it. It is self-checking, which is my favorite thing, and seriously saving my butt in das uber-class that now has 31 kids in it. I don't even have 31 desks. I just hope at least one person is absent every day. I am totally serious about this.
Make a worksheet of problems organized in two columns. Column A and column B. The tricky part is the pair of problems in each row has to have the same answer. Obviously some topics are more suited to this than others. (Solving linear systems, easy. SOHCAHTOA, easy. Graphing inequalities, hard.)
Pair up the kids. Decide who is A and who is B. Tell the kids to only do the problems in their column. When done, compare answers to each question number with their partner. And if they don't get the same answer, work together to find the error. That last step is where the magic happens. I know how well I taught the topic by how busy I am while they are row gaming it up. (Sipping coffee: go, me. Running around like lettuce with its head cut off: self-recrimination time.)
I'll also do this by projecting 2-3 pairs of problems for 5-10 minutes of practice at the end of a lesson. Row Game Lite.
Here is an Operations on Radicals and a Permutations and Combinations worksheet to get you started.