You need to keep calm, if you get excited or too nervous etc, (anything that gets your heart going, really) then you will wake up. If you feel things and focus on details it can keep you in a dream longer. Once I was in a lucid and it started fading, I walked twords the nearest tree (which had really rough bark, like a pine tree 4-5 foot in diameter) kept calm and felt its bark with my hand. I focused on what I was feeling, I tried smelling the sap and it steadied my dream within seconds.