Accepting your invitation: Regardless of how your invitation is received i.e. by post, email, telephone or simply verbally face to face. It's very important when accepting your invitation to do so in good time. Don't keep your host waiting so long that they have to chase you up for an answer.
This is just commonsense and common courtesy really. Leaving your host hanging on, waiting for your answer, can be very frustrating for them.
If you are not sure you can accept because of having to wait yourself for confirmation of another pending appointment, at least let your host know the situation, so they don't think you have forgotten them.
Keeping your host informed and up to date will be greatly appreciated by them and show that you are a considerate person.
The number one unwritten rule: Once you have accepted your invitation never ever just don't show up, or cancel last minute simply because you had changed your mind, didn't feel like going, or were not in the mood on the night.
The amount of time, preparation, effort and money that your host has invested in you as their valued guest is probably huge. Obviously, your host will understand if you have to cancel last minute because of a genuine, real life emergency. But barring that, cancelling on a whim, or worse, just not showing up, is just not done!
When you have accepted your invitation, contact your host by phone and ask if you can bring or do anything to help. If for instance you have a good reputation for doing great desserts you could maybe offer to bring along those to help out. Or maybe if you don't drink you could offer to pick up and take home some of the other guests.
If you're asked instead to help in some other particular way try your best to cheerfully accommodate their request and do it to the best of your ability.
More than likely they will refuse any help from you and insist that you just bring yourself.
Even so, the fact that you offered won't go unnoticed by your host and it all goes to contribute to how you are perceived as the perfect guest, helping to secure a possible future invite from them.
The same goes for when you arrive at the dinner party. Don't forget to offer your help. It might be greatly appreciated if your host is struggling a little for time in the kitchen.
Simply be polite and accept that your help may not be required, so don't insist on helping or make a big deal over it. Just relax, go with the flow and enjoy the dinner party.