[edit] Steps

When your sister/brother has her friends over (and that includes boyfriends/girlfriends), don't bother them. You might want to talk and hang out with them or "play" with them, but they deserve their time alone. Wait until they call you. If you start to hang out with them and she asks, "Can you get out now?" say, "Sure" and go and do something else. She will appreciate that and more likely want you to come back.
Never eavesdrop! Eavesdropping will get you nowhere. You usually want answers, and you'll end up right back where you started because your sister/brother will find out and you'll lose his/her trust.
If there is something small that your sister/brother did, don't be a tattle-tale and go tell your mom. Talk to him/her before you tell your mom, and if you can't work it out, then tell her.
Make sure they know you appriciate him/her. You can say 'Thank You' randomly or even do a favour and say you're doing it as a 'Thank-You'.
Never be afraid to talk to your big sister. She's there to help and no matter what, she will always be there for you. To be able to talk to her will make you trust each other very well. And it will keep you two together when there might be a time of difficulty for both or either one of you.
Be a shoulder to lean on. Both younger and older sisters need this. It will strengthen the bonds you share between eachother.

[edit] Tips

Some things just aren't worth fighting about. Choose your battles with your sister/brother.
If you want attention from your sister/brother dont order them around, ask for it in a kind way.
Don't lie to your sister. She can normally see right through it. And that will damage the trust you have for eachother.
"When mom and dad don't understand, a sister always will." ~Author Unknown
"You can kid the world, but not your sister" ~Author Unknown