Thought I'd jump in since I'm madly in love with my partner of 20+ years.
We don't have a 24/7 BDSM relationship, but when we "play" now that's another story.

When my husband and I play, sometimes pain can be a gift.
A little mind you, cause I'm not one for lots of pain, either.
But you've heard the expression - "It hurts so good."
Yeah, that kind of pain and everyone has their own threshold. There is a big difference between "slap and tickle" and "bruised and battered".
To each couple they find what they enjoy and want. One girl's bruise is another's proof of ownership.

Whether you turn your play into 24/7 or not, if you are finding yourself filled with guilt or your partner filled with fear, then neither of you will be satisfied and the relationship has gone past healthy. You are wise to be thinking about this and asking questions now.

That said, if you MUST punish her, because that is the role you two decide you want to have, then ask yourself are you punishing her for fun -
as in -- "You've been naughty slave."
"Yes, I have, Master, please, please show me the error of my ways."
Spank, spank, whip, whip, ending in lots of joy for both of you
or for real.
If then, please leave your anger out of the punishment. Time outs are good for both parties.

Communication is key.
What does she consider punishment and what play?
What about you?
Hard limits should stay hard.

Remember - not all punishments deal with pain nor should they.
If she likes pain, don't reward her with it.
If she doesn't like pain, you may not want to punish her with it, and slip into that cycle of guilt and fear.

Instead you can try other things:

1. Tie her up, tease her and leave her unsatisfied.
Just sitting next to her, watching her, and her knowing that she can't come for a certain period of time can be wicked.

2. No masterbating for a day, couple of days...

3. Taking away favorite toys, movies or privileges

4. Giving her too much pleasure...forced orgasms.

5. A few well placed clothespins - a few, not a hundred.

There's tons of ideas out there in stories, on other threads, etc.
More links to check out:

http://www.bdsmlibrary.com/forums/sh...sponsibilities -- Submissive vs Slave

http://www.bdsmlibrary.com/forums/sh...sponsibilities -- Dom vs Master

http://www.cuffs.com/whippingposts.htm
Another site with excellent articles and stories.

http://www.cuffs.com/davealexus.htm
BDSM love stories by Master Dave and Alexux
-- Read them with your partner for inspiration

I wish you the best as you blossom and grow into your relationship.