It should come as no surprise that children who've seen the first-grade curriculum before have a better chance of picking it up easily, so one of the best things you can do for your child is to introduce some of the material over the summer, before school even starts.
Find out who your child's teacher will be, and talk to him or her about what your child will be learning next year, along with what skills are expected from day one, since touching on both kindergarten skills and first-grade ones can ease the transition. Addressing some of the old and new reading, writing and math concepts, in particular, can give your child a real boost come show time.
Don't forget to make it fun, though -- it is summer, after all. Games, contests and play-guided lessons will hold your child's interest much better than strictly academic ones.
Next, because familiarity is your child's friend ...