Exactly what TAAVS said.

You have normal hours, weekend hours, and after hours. The weekend hours can be billed at 1.5 - 2.0x (whatever you like). And after hours can be billed at 2.0 - 3.0x. No way you are not going to make good money and be worth it, if they have an urgent request.

Also have in writing what the response time will be. 48hrs., 72hrs., etc...
If the client wants quicker response times than that, make it worth your wild. Define how much extra a response time of 24hrs. will be. Same day service or even same day service no matter the time of day.

Example: Customer calls on a Saturday evening and has to have something done that night. Weekend rate + after hours + same day response = 8.0x regular rate.
If you are at $100 per hour, then he is paying $800 per hour that night for you to rush out there and do whatever.

They can either say yes or no to these terms. But they are your terms and they are not unreasonable, when unreasonable requests are made.

[edit]: With the retainer, you would also need to set what amount you need to get to cover anything that might happen within a month's time. This would depend on the size of the customer's system, how old it is, and expectations of the customer.
You could be looking at anything between 10% of the installed cost of the system - to as much as 90%+.
If customer demands service within hours and guaranteed up-time (meaning instant replacement of any failed component); then 90%+ since you would need to stock 1 of everything in the system as well as be on call all the time.

[2nd edit]: Service contract would be completely different. As the month fee would have to define a set amount of time & service allowed. And terms of what would be considered extra and billable, would need to also be defined.

[3rd edit]: If you get more than a few of the VERY demanding clients and they are paying the extreme fees. You will need to hire more techs to deal with the over flow. That is very expensive and why the fees to deal with these kind of clients is very expensive. Otherwise you are shooting yourself in the foot.

Usually you will not run into these kinds of clients. At least not once they see the amount of money it costs for what they want. As has been said many times "There are no real emergencies when it comes to someone's TV".
But there are clients that will look for an idiot to take advantage of and get free or above & beyond work from. Most of us have fallen into that trap once in awhile.