It's certainly easiest to sleep with Michelle, but if you screw up and don't take her out before you lose the ability to date her (like I did), try Carmen Ortiz, the SoBoHoe. It takes 2 dates. You get her number on the first, and sleep with her on the second. Prep -
- Buy 2 suits from Perseus. If you're done with the story, you should at least have 1 from story missions. I used the obsidian and the charcoal.
- Get a nice car and park it at the South Bohan house. I used a Cavalcade.
Go to Tw@t and attempt to date Carmen on love-meet.net. 12-24 hours later, you'll get an email back. Go save a couple times at the nearest house to advance time, then wait around for a response. Receiving the email will put an appointment in your phone. Carmen's 1st date will always be at midnight, and her place in South Bohan will pop up on your map at 21:00-22:00. If necessary, go to the South Bohan house and save the game until the timing is right. Switch to one of the suits, grab the nice car, pick her up, and go to the strip club, which is close by. At the end of the night, she will give you her number. Wait a day or 2, call her, and repeat this same date with the other suit. She may complain and say something like "What is it with you and strippers?", but it doesn't matter. Try your luck at the end of the date, she will let you in. She also liked me enough to unlock her special ability--you call her and get a health boost.
I personally haven't verified this, but I've read that if you're going to play the Gay Tony DLC, you can also get the achievement via booty calls -
http://gta.wikia.com/Booty_Call - This may be even easier than dating Carmen.