There are many things to consider when using the HAT.
I am by no means a complete expert but I have learned a few things about it that will probably help you. As far as my experience, I was an avid HATter (but not mad

) in 0.5. I skipped 0.6 and have been playing 0.7 since release.
As far as getting used to the 0.7 HAT, the tips above are very good. It does take 15 seconds before you can bring the sights up, so make sure you have it equipped most of the time. It is also true that you cannot fire from prone so crouched or standing are the only options. Of these two I prefer standing and I will explain why below.
There are several other considerations for the HAT.
1. Range – The sweet spot range for the HAT seems to be between 300 and 600 m. Beyond 600 you can not control it as well and inside of 300 can just be plain dangerous.
2. Ammo – You have 2 rockets. This is enough to take out a tank if you get two rear shots. You can also drop any lighter vehicle in one rocket. Side and front tank shots will take more rockets. Also note that a bunker or firebase can be mostly destroyed (as in back to the foundation) with only one rocket (I have not tested other static assets yet.)
3. Aim time – It takes a while to get the reticule to settle down on the launcher, be patient; if you jump the gun you will miss (or possibly kill yourself/teammate.)
With these things in mind there are a few things you can do to maximize your potential.
You will want to stay with squadmates as much as possible. They can provide you cover fire, ammo, and extra eyes for taking out enemy armor. This will allow you to stay in the battle longer and to engage armor faster as you can keep your launcher out.
If you have to venture on your own with HAT it is a whole new ballgame. Many will tell you to never do this, but it can and sometimes MUST be done to maintain a low profile for the team.
The biggest thing to do when you are solo HATting is remember you have NO support. This may sound obvious but when you are in a team oriented, (and more specifically squad oriented,) game such as this; it is easy to forget when you do NOT have backup.
Get familiar with your pistol. It can save your life. You will not win many head on engagements, but with surprise you can drop another person and may be able to get away.
Ammo will be a huge problem when you solo the HAT but it can be dealt with. Firebases and Bunkers are a great (and usually only) source of ammo when you are at it alone. Remember that if you hit a firebase or bunker it maintains the foundation and you can reload off if it. This means hitting an enemy firebase when it is empty can be a sound strategy to get a decent rearm point (just do not stay too long or you will be found and killed.)
Whether you are on your own, or with a squad, cover is your friend. By not standing out in the open you can significantly reduce the odds of being seen and landing that critical first shot. Not all cover is good however. Try to think like a tank, think about where the first place you would look is and avoid hiding there. Rooftops, giant rocks and small tree areas all scream “put a giant HEAT round here,” so avoid them like the plague. I have found the camouflage works well in this game (specifically this release) so being halfway up a hill and such can afford you just enough time to land that shot.
When aiming the HAT, I tend to aim JUST ABOVE the vehicle. This will not give them a lock-on tone and you will be able to settle the sights in relative safety. Once the sights are settled, fire the shot and move the mouse slightly down so you are on target. This will give them as little warning as possible before the rocket hits.
The last big thing I want to touch on is to be mobile. Remember how I said I prefer a standing stance to a crouched one? Well the reason is because every second matters. Once you let off a rocket everyone and their brother will be able to find you. The rocket leaves a distinct smoke trail that will be traced back to you soon. This is most apparent against tanks who take multiple rockets to destroy, but is also apparent with APC’s and the like because they usually have supporting troops. It is good to take out the armor, but it is better to take it out and survive to do it again. I find it easier to bug out after a shot if I am already standing.
Never fire from the same spot twice unless you are 100% sure that nobody saw you (which is almost never… so just do not do it.)
Also, NEVER just sit there and lock onto a vehicle. The vehicle will initially freak out and move around; but it will eventually start to open fire in the area. This is bad because a shot could kill you. Even if the shots do not hit you, most people will bug out and run when the shots start to get close. This means that the lock-on will go away and Mr. Vehicle will have a very good idea of where you are now and concentrate the fire. Even if this is not the case, an experienced vehicle whore will call for infantry backup in the event of long lock-ons.
This is by no means a complete guide. This is just my insight and personal experience regarding HAT. Feel free to comment on it and let me know what areas I am wrong on or could use better explanation.