All of Colas' suggestions are very good. But if you find you still can't paddle over the top of the wave (or find a rip or get out through a lull), here are some other suggestions for getting out the back.
** I do have a theory and I don't know if others would agree - I would rather my leg-rope was tight (i.e. stretched a little) when the wave hits the board rather than allowing the wave to hit it, yank it, accelerate it, and put more force on the leg rope (and your leg and anything that may accidentally get tangled (feet, toes, etc.)). I feel that reduces the risk of injury and the risk of the leggie breaking.
+ Paddle toward the breaking wave and then dive under at an angle - if the wave is at 12:00, at the last moment paddle towards 2:00 and dive deep and long under the wave toward 10:00 (as long as there is no chance you will hit bottom). This ensures the board won't come back and get you, and you make progress under the water, and it causes the leggie to straighten before the wave hits the board.
+ If you are getting hit by wave after wave (and you are not ready to give up), then get to your feet, get in a paddle or two, and dive forward as above. Every few feet of progress can help - it can put you just far enough forward so you get over the next wave.
+ You can do the same thing on your knees. Let's say you recover from one wave, quickly get to your knees, paddle a 4-5 times towards the next wave, and then turn the board towards 2:00 and dive from your knees towards 10:00. (Some people say that paddling on your knees is a no-no, but sometimes it is all you can do to make progress towards the next wave.)
+ Try to paddle over the top (if it isn't
too big), but kick the board towards 2:00 while you dive towards 10:00
if you know you can't stay balanced - it stops the board from being dragged back by the wave, and ensures the board does not kick up and hit you.
+ If you can't get to your knees or feet before the next wave will hit, turn your back to the wave, get the board fins-down, hold the paddle in one hand (
and don't let go) and the rear of the board in the other (pointing towards shore). Just before the wave hits, use the hand on the end of the board to lift yourself up as you push the board away towards shore - and you go down under the wave. Reasons: the leggie will be stretched before the wave hits; you will drop under the wave and won't be pulled along as much; the board will often stay fins-down and won't tombstone (dragging you towards shore); it will be easier to drag the board back toward you so you can get back on quickly. But remember, beware not to get your fingers, toes, feet, ankles, etc. tangled in the leggie.
+ When you get the board back you need to learn to get back on quickly. Lay your paddle across the board at right-angles so the handle is on the right side, with the paddle in the water providing lots of stability 0- see my crappy sketch below. Put your lead hand on the paddle shaft and place your other hand on your board then climb up -
always keeping downward pressure on the shaft - you will be amazed how much more stable your board will be. Keep your hand on the shaft as you stand up, then grab it and stick the paddle in the water ASAP - it will increase your stability. This will get you on your board quickly in water that can be turbulent after the wave has just gone by, and you will be stable so you can start paddling quickly.
Sorry for the essay, but I have been doing this a LOT lately (surfing at a beach break with no rips, etc.) and it has really helped me get out through 100 m + of pretty good size breaking waves.