Describe your band. This gives context to the subsequent information in the description of suitable bass players, enlightening them about their potential musical home. For example, "rock band seeks bass player" communicates a very limited amount of information. But "Hard hitting, melodic power-metal trio seeks dedicated and passionate bass player." From this brief sentence, you communicate information about line-up, sound, style and attitude.
List your ideal bass player's influences, and describe the type of player you're looking for. Talking directly to the person via the ad enables them to engage with you. Use your own influences to give the bass player something to compare with his own. This is where you paint a detailed picture about your sound and stylistic ideals. Don't list everything --- use bands and artists that represent the breadth of your influence spectrum. For example, "You can slap like Flea, groove like Noel Redding and tap like Billy Sheehan. You also like Tower of Power, Television and Blondie."
Include detail pertaining to responsibilities. In the interest of fairness, it's essential to lay out in advance what you expect from your future bass player. Issues such as rehearsal frequency, tour commitments and whether or not he will be included in the songwriting process are important to any person considering auditioning for a band. For example, "You'll be available Tuesday and Friday evenings for rehearsals, where you'll always have a ready supply of new riffs and ideas. You're be able to leave town to tour on short notice and are prepared to help with driving."
List personal attributes. As well as musical information, your ad must give a sense of what kind of personality your are looking for. Avoid mentioning political and religious issues in the ad so as not to alienate anyone unless your music is linked to your religion or politics --- for example: "Christian-rock band seek like-minded bass player for Republican fund-raiser." Otherwise, focus on the things that you consider to be "deal-breakers" --- for example: "Easy-going, dedicated bass player with professional attitude and willingness to adapt their style required for funk band."
Write a couple of lines about logistics. This is where you cover practical issues, such as "you have a clean driver's license and can get to downtown Dallas for practice."