M.T.
I had my children extremely close together but the best advice I was given was to include your oldest in everything w/the new baby. Let him help change her, whether its getting diapers, wipes, let him help get bottles out of the cabinet, just make him feel like a big boy who is helping mommy take care of his baby sister. I would even recommend if you are nursing, include him in that as well. Let him watch and explain that is how the baby gets her milk. One thing we did prior to having our second was let the oldest pick out things for the new baby coming whether it was t-shirts, blankets, toys, etc. The main thing is to not see his acting out as him intentionally being naughty but that he is just trying to figure out his place w/this new addition. If he see mommy spending a lot of time w/the new baby changing diapers, feeding, etc. then he will do what he can to compete for the same attention even if it means terribly regressing in his own potty training. I hope this helps. Remember no matter what enjoy your new family. God Bless!