So you've seen a listing that catches your eye but you're not sure what to say or maybe you've never answered a listing before.  Here's a list of tips to get you on the right track and avoid making the most common mistakes.  This list can apply to anyone answering a 'Musician Wanted' listing.

Buy all the equipment before you answer the listing
Buy and be familiar with all of the essential items to meet the expectations of the band.  This doesn't have to be thousands of dollars worth - but you should be able to play a medium sized venue without borrowing your friend's amp. 
There are obviously exceptions to this rule if the listing specifies they have a backline, but you're still going to have to practise on something at home so you shouldn't consider this a 'get out of jail free card'.

Communicate like an adult
Use clear, concise communication. Reply frequently and be sure to answer all their questions, no matter how small.  This indicates your level of commitment to a band and also the level of communication they would expect to receive if you're asked to join the band.

Know the level of commitment that you have
State how music fits in your life, the number of commitments you currently have (full time job, family, other bands) and how much time you are willing to put into a new venture.  If you're contacting a full time working band then they're going to have to know if you're just a weekend musician.

Treat every ad as if it specifies 'NO TIME WASTERS'. 
If you only have a passing interest in a project while you're between jobs or your girlfriend is on an overseas holiday then don't waste anybody's time.  If you could never match the passion that the other members of the band have, then politely step back before you annoy anybody.

Be honest
Be honest to the band about the gear you have, your level of experience, your access to transport and the range of skills that you have.  This also extends to being honest with yourself.  Practising once a week between your 6 other hobbies is not enough to learn songs and contribute to a working band.

Let them know if you change your mind
If at any point you realise the band or the situation isn't for you, TELL THEM. Dropping communication completely is never going to gain you anything, a simple 'Thanks, but no thanks' frees them up to decide on other applicants.  Politely withdrawing may leave you as a friend of the band; perhaps you may be able to help each other out during recording or gigs, or maybe join the band at a later stage.  Don't deliberately get yourself a bad reputation, musicians know musicians.

After you've got yourself an audition, head on over here to read advice on how to audition for a band.