If you are looking for LGA long term parking you should never use the official airport terminal parking. Instead you should look for the best deal you can get in one of the off-airport parking service providers.
Each off airport parking provider has its own ground transportation to the airport itself. There are many LaGuardia airport long term parking companies which are all very much similar: you arrive to their parking lot, drop your car there and catch a bus or a shuttle to the LGA airport. The shuttle will drop you off exactly where you need as it goes around the whole airport.
As you can understand by now, it does not really matter which company you choose as they all provide the same service. The only sole difference you should look for is the PRICE! Each parking provider has its own offer / deal / coupon almost all parking lots have various parking rates in the same time period.
Another option worth considering is a local hotel, many hotels offer free parking with even one night reservation but usually cost more money.