What is consent, really?
Consent is simply an affirmation from anyone involved in a sexual encounter, that they want to be involved in everything that is happening/will be happening. In order for this to really mean something they can't be in a chemically-altered state (because alcohol/drugs can alter one's behavior and even leave them with little to no memory of giving consent in the morning) and they cannot feel pressured to say yes (this has to be THEIR decision.)
A really good analogy that I've heard for consent is that it is like the water in a pool: you need it to be there when you dive in, of course, but you also need it to be there for the whole time that you are swimming. If someone starts feeling uncomfortable and withdraws their consent in the middle of an encounter, that encounter has to stop right away (just as you'd stop swimming if the water was suddenly gone!) This might sound like a buzzkill, but consent is important because it is what allows you to be sure that you are not taking the risk of hurting your partner(s).
How do I get consent without ruining the mood?
When you think of consent as an enthusiastic affirmative (silence is not enough to gage consent!) then, suddenly, it becomes pretty awesome. Don't you want to have sex with someone who is telling you YES, I want that and YES, I want you?
What would feel best for you?
Tell me what you want me to do for you...
How about I do x to your y right now...
These are just a few examples of sexy ways to ask for consent! The bonus of all this is that it gets you talking to your partner(s) about sex which will allow you to discover new things about each-other (like what you like and what you don't like) that will make your sex even better!
The Yes Means Yes Blog is also a great resource for this topic.