1. Before buying a sewing machine, try before you buy! I love Sew...Northampton for this reason. They let you play on any machines for as long as you like and they really listen to your buying specifications and don't do the hard sell on you.
2. When you've purchased your sewing machine, make sure you give it a regular MOT by taking it for a check-up at your local sewing shop. Again, Sew...Northampton do this for me. They even got my first, basic sewing machine working again :-)
3. If upgrading or replacing your sewing machine, read up on reviews first and ask those long time sewers their opinions. Twitter is great place for seeking advice.