Research your dog before buying him and then train it properly

Researching your dog for energy is important.  The dog's energy has to match the family's energy if you want to be happy with your dog.  There are 4 energy levels.  Very high such as in a Chinese Crested, High such as a Vizsla, Medium such as in in St. Bernard and low such as in a Shitz Tzu.

Think of the dog first as an energy level, then think of the breed, otherwise both you and your dog will be off to a bad start.  The dog's energy is probably the single-most important characteristic that you must deal with.

This is a good site with 20 important questions about the dog's activity.  Select a dog

Cesar Millan, The Dog Whisperer has some good thoughts about the dog's energy and you. Dog's Right Energy

Training your dog is important too.  Avoid just buying a dog and trying to be the trainer.  If you don't have the proper techniques, you will certainly be struggling with your dog.  Get you and your dog off to the right start by taking obedience classes and practice at each occasion you have, 5 minutes at a time.  The following DVD will complement classes to give you a deeper understanding of the process.

I highly recommend the DVD "Living with your Dog" by Jack and Wendy Volhard Living with your dog.

Cesar Millan the Dog Whisperer has a set of 5  DVD that are complementary to the Volhard's.  Worthwhile getting at: Cesar Millan DVD on dog training