If you decide to spend some time in Miami, it's fairly likely you are going to end up in South Beach. If you end up in South Beach with a car, you might have a problem.
The solutions...
Parking Garage
Spaces in parking garages are convenient and plentiful. Between 23rd Street and South Pointe there are over 5,000 parking spaces available in city parking garages. This figure does not include private parking garages or city and private surface lots.
City parking garages charge 1 USD per hour with a maximum of 16 to 20 USD per 24 hour period.
PDF download - City parking garages, Miami Beach
Meter parking
While meter parking is available on many South Beach streets, it is pretty tedious and often ultimately impossible to find an available space.
Valet parking
Some South Beach hotels have their own parking garages, although capacity is often fairly limited.
Valet parking stalls are often set out kerbside, many being run by third party parking companies contracted by the individual hotel. It appears that the valet companies park vehicles in the nearest city or private parking structure.
The usual valet parking rules apply in terms of convenience and cost. Expect to pay upwards of 20 USD for a 24 hour stay.
As with every time you park, do not leave valuables on display. We were only in South Beach for a relatively short time, but we still heard stories of vehicles being broken into and laptops and passports being stolen. Exactly, go figure!