There are two aspects to this: getting out, and then getting
back in once you're out.
1. Prior to hopping into the water, make sure you check out
where the waves are breaking the most and least. Obviously you want
to paddle out in the easiest position (unless you are out for some
carnage), so, conditions permitting (rocks etc.), paddle out in the
calmest area. For example, at a point break, always paddle out from
the middle of the beach in the area where the point waves do not
break so strongly.
Once you're going, it is important to have some momentum when
approaching a broken wave. If you do not, the momentum of the wave
will backshoot you.

In addition, as you are about to go over the wave, lean back to
allow the nose of the ski to lift up over the wave more easily,
otherwise you will tend to plow straight into the wave and get
pushed further backwards, or backshoot again. However, there is a
limit to how much you can lean back - on small waves you can lean
back as far as you want, but on larger broken waves, you can't lean
back too far as the momentum of the broken wave, combined with the
lifting nose, can potentially cause your ski to do a backward
somersault. Not fun!
If the white water is simply too big, roll your craft upsidedown
and hang onto the foot straps (and your paddle) - the wave will not
push an upturned craft back as strongly as an upright one. Once the
wave(s) has passed, you can jump back on and continue on with your
journey. Only experience will tell you when to make this judgement
to roll.
Try to recommence paddling as soon as possible once you go over
the wave so that you can keep forward momentum going. This will
also allow you to re-establish stability sooner.
You may not paddle a straight course out. If you have room, be
open to moving sideways to avoid broken water where possible.
2. Getting back in can be tricky depending on the size of
the waves. The most important rule of thumb is to always keep
paddling on the wave (particularly once it has broken or is about
to break) so as to keep your craft out in front of the wave. This
prevents the wave from interfering with your rudder and skewing the
craft off sideways.
Many paddlers often stop paddling and lean back once they catch
a wave to prevent them from nose diving. Don't do this. Paddle down
the front of the wave and this will get you out far enough to not
nose dive. If you make a late take off and the face is very steep,
by all means lean back, but don't stop paddling while you do so. If
you still nose dive, you simply took off too late and were going to
nose dive no matter what.

In this case (and only experience will teach you how to judge
this), back off and catch the next wave. Sometimes this
requires back paddling or throwing the legs over to put the breaks
on. Whatever is required - it doesn't matter.
If you do go sideways, or spong, lean in to the wave and try to
wrap your outside arm (to the whitewater) around the ski so
you keep your ski with you and you get taken in with the wave and
ski. This is not an easy skill and will take time to learn.
If you are good at all of this and are ready to start hot
dogging on waves, then you don't need my advice. Just get out there
and you will learn the tricks. But never forget the golden rules
above on race day, as you won't be hot dogging then!! Unless, of
course, you want to entertain the crowd.
So there you go - it is really quite simple (NOT!) It does take
time and practice to master the surf, but it is fun once you
do.

One final tip for both getting in and out that will save you
buckets of time is "never lose your ski or paddle". Do whatever you
can to keep them. This might mean hooking your feet under the
footstraps and being dragged by the ski for a while, or grabbing
the footstraps (with paddle) and rolling and getting dragged. We
all get wiped out occasionally, and swimming after your ski or
paddling with hands because you lost your paddle absolutely suck.
And they waste serious time!