Birth control

I am married, 24 with 3 girls & I will be starting school in August, my husband & i have agreed we want no more kids for the next few years, my question is what is a good birth control. I have been on pill & depo & gained over 70 lbs with it :( Right now we just do withdrawal but i get kinda nervous i could end up pregnant accidently. :/ My youngest is 3 & i would hate to start over especially with school coming up. Any suggestions??