Usually, I have found that taking a step back and understanding that the startup is a part of your life, and not your entire life (even though it may feel like it consumes you),helps in preventing burnout. Having friends and family around or even a hobby can help. The other tip I have discovered is looking at the burnout as a problem solving exercise. Think of why everything seems hard, why you're not motivated, figure out which parts you can solve and which you can't, and solve the parts you can, piece by piece.