The Root of the Behavior
A lot of this behavior can come from boredom. They may sit on you or your head simply because they like the way you react to it and they simply have nothing better to do. If this is the case then supplying them with some toys or other things to occupy their time may find you the relief you are seeking. Often chew toys, bones or stuffed animals work rather well, though there is a large variety of toys and your dog's preferences can vary. Take note of how you typically react when they sit on you. Do you get really animated, perhaps chase them? Though you are mad or frustrated, your dog may not see your reaction as angry. For instance, if your dog sits on you and in reaction your get up and yell and chase your dog, then instead of coming across as angry, this may come across to your dog as a game. Sit on their owner and they will get up and play chase with you around the house. In this sense, it can be considered a learned behavior. They learned to do this because you react in a way they consider a game. So even though you think it is frustrating and annoying, they believe you enjoy it and that you also want to play this game with them. Each time they sit on you and you react in this manner you are reinforcing the idea that this is a game and breaking this form of cyclical reinforcement of their behavior is your first step to changing the behavior itself. Dogs unintentionally learning unwanted behaviors like this is remarkably common. Dogs primary form of communication is actions and body language while humans rely on vocal communication. When these lines of communication get crossed, dogs often walk away with the wrong lesson learned.
Encouraging the Behavior
As it is often a learned behavior breaking the cycle that reinforces that behavior is your first step. If you typically chase them around the house when they sit on you, stop doing that. After while they will learn that you will not initiate this game with them because they sit on you, and that will remove the incentive to do it in the first place. However, they may just be sitting on you because you are the warmest place to sit. Separating out an area for your dog to hang out and call their own is going to do a lot to adjust this behavior. Lay out a dog bed, put their favorite blanket and a few of their favorite toys will make this area feel like home and make your dog choose this area to sit instead of sitting on your because your warm. Smaller breeds often sit on you because they have some form of separation anxiety and being as close to you as possible is always their goal. A behavioral specialist may be your best bet in this case as they can diagnose the root cause and give you the path to correcting this behavior for your dog.
Other Solutions and Considerations
If they are simply seeking a warm place to sit down then providing them a dog bed in a warmer part of the house may help. Placing it directly next to a vent may be a fire hazard, but relatively close may just provide a warm place they enjoy. If the behavior persists then a dog trainer will be able to correct the behaviors using command signals. This is a relatively easy solution and will give you the ability to simply tell them to stop and they will. Either way, paying attention to how you react to your dog when they exhibit these behaviors will give you a better understanding as to why they are doing it. Often once you know this, you can easily correct unwanted behaviors yourself.
Whether they think it is a game, they want a soft and warm spot to sit, or because they just need to be around you all the time, there are definitely things you can do about it. Sometimes dogs can just be rude whether they intend to or not. A few best practices around the house can often quickly change this behavior, but all in all it is not something to fear and does not typically indicate anything physically wrong with your dog.