How do I prevent myself from regaining the weight I lost?

I started eating healthier and controlling my portions and I lost 30lbs in under a month while going on a run and doing yoga in the mornings before work. However this week I lost 3 pounds but gained back 4. I do not want to gain the weight that I had lost and I want to continue to lose weight. If anyone could give me some advice on how to keep off the weight while continuing to lose weight then that would be extremely appreciated :)

~Robbie