Internal rewards are ones that involve only you and what makes you happy. Be careful not to go off the deep end here, though; especially if your heart's greatest joy is ice cream (I know mine is). Try to find ways to celebrate that don't involve undoing the work you've put in all week.
I highly recommend professional massage therapy after you've been faithful to your plan. You and your budget will determine how often it's appropriate to reward yourself this way.
External rewards involve something outside of you. Here's an example. When I worked at our local children’s hospital, every week I met my workout goal I would go to the gift shop and buy a balloon or small gift for one of the kids in the cancer unit.
Those kids are there through birthdays, holidays, and sunny days made for play but they can’t go home and some of them can't even go outside. Not to mention the hardship they have to endure of fighting for their young lives every day.
I would stop at the nurse’s station to find out which kid was having a particularly hard time and pick out something appropriate for their age and gender. The joy on their faces and the way they lit up when they got a gift from a total stranger was enough to motivate me to stick to my plan. I knew someone else was counting on me to meet my goal even though they had no clue.
Find your own way to tie your progress to doing something good for someone else. You will reap rewards that go a lot deeper than how great you look in your "skinny jeans."
If you like to write, keep a journal. If you're a compulsive planner, make notes in your calendar or Blackberry. If seeing is believing for you, take photos along the way. Do whatever is going to stimulate you to keep your weight loss train moving.