I have to force myself. For me, it's easier if I first pull fabrics for the quilt top, lay them out so that I think they kind of go together, and then just start cutting. I don't choose a backing fabric until the top is finished, and by that time I'm so eager to get the quilt finished it's more a matter of "this fabric will work" or I'll have to go and buy something that fits.
The other thing I do is reassure myself frequently that there are wonderful fabrics out there I can buy to replace the ones I have. They don't have to be exact replacements; they just have to be equally wonderful!
I also have found that it gets easier with every quilt. The more quilts under the belt, the easier it is to start a new one! I think that's because I realize that there is no need to make only "perfect" quilts; it's only necessary to actually make a quilt!