In windsurfing the harness makes all the difference in speed, comfort, control and obviously endurance. Once you are comfortable with stronger winds it's most important to get yourself familiar with hooking in and out.
Choose a harness that has a snug fit, a seat harness or a waist harness is fine. The first few attempts can be scary and you may feel out of control, as if the sail is controlling you. That is why it's a good idea to start with a waist harness because it's easier to hook in and out. When you feel freaked out you can quickly and easily unhook, giving you back that sense of control.
Next, you'll need to have the right harness line length and at the right position on the boom. Your boom height is also important as a boom too low or too high is no good. If the boom is too high, it will be harder to hook in and out and if the boom is too low, your arse will be dragging in or skipping on the water.
A good way to test this is to climb on your board, lift the sail and check in relation to your hook where your harness line is hanging. If the harness line hangs below your hook then either your lines are too long or the boom is too low. In general and when standing on land, your boom height should not be above your shoulders and definitely not by your belly! Over time you'll find that sweet spot, but for now a good height (when sailing) is for your boom to be just below your shoulders and in line with the crease of your armpit. Keep in mind, this should be with your sail on the board because the mast base makes the sail higher than when on land.
When it comes to harness line length it gets controversial here, some say it's good to start with long lines as it helps with hooking out easily. However, you should start with the correct length from the start. That is to say, the length you start with is the length you'll die with. Using longer lines changes your entire sailing position and you'll start to form nasty habits that will be hard to shake off. Adjustable lines are a great start, but if you follow the above in terms of boom height and relation of the lines to your hook, you'll easily find the right length.
The lines need to be positioned on the boom in such a way that the pressure is even on the back and the front hand. If you let go both your hands then the sail should be balanced by using your hook alone. It doesn't have to be perfect, but it should be feeling light in both hands. If you let go one of your hands the sail should be stable and controllable, it shouldn't whip you forward or back. If you find that while sailing your front arm is getting tired move the lines slightly forward, if your back arm is getting tired move the lines slightly backwards. Moving the lines back and forward makes a huge difference as the sail is very sensitive to this. The line width between the straps should be around 10cm to start with, as you become more comfortable you can change this to fit your style.
Try hook in and out a few times while standing with the sail on land. It's often easier hooking in than out, hooking in and out requires you to lift your hips, shift them forward and pull the sail towards you slightly. Only hook in when the wind is strong enough to do so, it's difficult to unhook when standing completely upright with the mast between your feet.
As the wind gets stronger bend your back foot, this helps with control and it keeps your weight back. You can straighten your front leg with your back leg bent, this will help you lock in as you take off. You should also learn to hook in before you start planing.