Obviously it is best to get one of the same species. Lacking that then at least the same complex. And when choosing the mate find a smaller one, as Repz said, but also make sure he's not the biggest one in the tank at the store. Usually if they get them from a supplier the stores have all juveniles but it doesn't hurt to ask. Sometimes they'll get a return of a mature clownfish.
Use extreme caution if you have maroons or one of the tomato complex (cinnamon, tomato, red/fire, et. al.) as if they don't accept the mate they might kill him. Especially the maroons.
To 125mph's point, if you put two clowns in a tank together, assuming they are male/female or one or both are juveniles, they have no choice but to mate. It's their instinct. This is why you so often get siblings from the same hatch mating up. They have no choice. It's also why you have the occasional mixed species like the maroon ocellaris.