How do you wire a plug that has constant power on the bottom plug and the top plug works with a switch?

Many outlets have small tabs between the terminal screws that can be broken out with a needle nose pliers. You can completely separate the continuity of the top outlet from the bottom outlet by breaking both these tabs and wiring each outlet as a completely separate circuit. The ground plug is the only line that cannot be separated this way, and will remain common for both outlets. Alternatively you can break a tab only on one side and have the outlet share either the neutral or hot line. For a outlet set up to have constant power on the bottom break the tab on the neutral side, wire your hot line as normal and the "constant" neutral to the bottom neutral terminal, then run a second neutral from your switch. This would allow for the bottom outlet to be live all the time and the top to be switched on or off.