Time frames and rewards for meeting milestones??

I'm curious to know how many people have set milestone goals, and if so, how successful you have been in reaching them. Additionally, how did you decide on realistic milestone goals? Did you do some sort of calculation, or just arbitrarily choose a number and time frame? I want to set milestones to encourage myself, but I don't want to inadvertently get frustrated instead if my milestone goals turn out to be unrealistic. I'm also looking for suggestions on how to reward oneself for meeting a milestone, when money is a bit scarce. In the past I've bought new clothes, but that won't be an option for me for awhile. What have you all done to reward yourselves? Thanks!