Doing your own landscaping and lawn work can be a very rewarding feat.  Exercise, fresh air and a sense of accomplishment are only a few of the positive outcomes of doing the work yourself.  Don't be selfish though!  Invite your spouse and kids (well, if they anything like I was when I was young, you might have to force or give a little compensation in addition to the invite...) to join in as well.  Nothing compares to the bonding that comes from pulling weeds and triming the bushes.  I think that enough of my advice... check out these great sites for some valuable advice on doing the work yourself:

The Basics:
Advice from Lowes
Organic Lawn Care
Cornell University Advice

Solutions to Lawn Problems:
Scott's Solutions
Bayer Solution Center