Really depends on my personal life. I try to set myself a goal for each day or week. Plan out models and how complex I want them to be. I think it's all about planning ahead and thinking it through. The main factor is finding the time everyday to do it, as you can see I'm behind 2 models because of my job and family.
Though it is pushing me to design-model-UV-texture-export models within a time frame which I think is good practice. Last it's all about dedication. As long as I want to do this project it will be done...and if I don't want to do it anymore then I've failed. Not sure about you, but when I fail something I just hate it, I feel I have to make up for it, do it again better. I'm still uncertain on where this project will eventually end...should be fun.
Edit:
Also I want to note that before I started this project it had been over 2.5 years since I last modeled something. I was a member on this forum then and I was constantly told I was a good artist...but I never actually finished anything. It was like false praise for half finished junk in my mind.
I had to get away from it, let a lot of people down. Recently I wanted to get back into creating games and media, but I made a promise to myself to actually finish what I start.
So this project is not only a test of my skill and a learning experience, but a test of my promise that if I finish will finally make me feel like I accomplished something.
..sorry for the ranting...:p
Good luck with your own portfolio building!