I keep premixed Bacardi and Coke on tap #6. How I do it is:
1. Add 2 gal Bacardi and 2 qt Coke syrup to keg.
2. Top up with filtered water from fridge dispenser (@ aprx 50*F)
3. Set regulator to 60psi, connect gas and shake end to end for 5 min.
4. Disconnect gas and set reg to 35psi. Give keg a shake to reduce head pressure and reconnect gas.
5 put keg in keezer and connect liquid line. Wait ~2hours for keg to chill, and serve.
It takes about 36 additional hours to reach full carbonation from this point, but the drinks on day one are about what you would get in a bar. The first day the drinks are a little sweet, but the carbonic acid from the co2 levels it out after 24 hours.
The really nice thing about this is that, since the rum is carbonated too, the drink stays carbonated the whole time you are drinking. Also, since you are not adding warm rum to cold soda and ice, the ice melts much slower and your drink doesn't get watery.