After hours parties are tricky since they are often unplanned and not the main event of the night. To that regard, wearing what you wore the rest of the evening is usually appropriate. If you are starting the night out at an after party, try and figure out where the rest of the party went first and then dress appropriately for that location.
If you are still at a loss for what to wear, consider wearing dark dress pants, a button-down dress shirt, black shoes and matching belts and socks. A sport coat could be included, but most people will probably ditch it by this hour. Additionally, trendy jeans can replace the dark pants in most situations.
Focus your outfit on where you plan to attend before the after party
Don't worry too much since it will be late at night and most people won't care what you are wearing
You'll never get turned away for looking too good