Make sets of fish crackers at snack time...  Use marbles, rocks or anything else around the house for practice.  Just be sure that the set pieces (in both sets) are all the same, so it doesn't cause confusion.  For example, don't have one set be rocks and the other be marbles... Skittles and M&M's work too, but I'd save these for a special math reward.

Remember how to set it up and do it:

• Make two sets of the same kind of object
• Count how many are in each set
• Write out the problem (like 2+3=) << optional step
• Put the sets together
• Count how many are in the final set
• Write down the answer << optional step

It's a good idea to keep the numbers small when working with sets...  Just use 1-10 for each set.

The goal is to get the basic idea of adding two sets together.  (You can even do three or more sets when your child really gets good at it!)