Be as descriptive as you can. Say what the item is, where it's from if it's ever been worn, and how much you originally paid for it!
Add items to your closet frequently. Don't add all your items on day 1. Add about 5 items per days so it looks like your "store" or closet is always growing.
Don't sell knockoffs. Simple.
Don't sell cheap or overly worn items. Poshmark is best for items over $15 that are in great condition and are fairly up to date and trendy.
Share other closets. Share other people's closets in hopes they will share yours, make sure to share a closet that sells items that you don't. Don't share the competition.
Take great photos. Take it seriously. Make your closet look like you're excited and like it's a real live boutique because it is. So take quality photos!