Be careful not to create too much glare when placing outdoor lights, think carefully what you are going to expose. You may have a dirty old shed somewhere in the corner which you don't want to expose at night, so think about the beam direction and where it is going to. If you have some lights at the bottom of the yard then make sure they are not pointing directly back at the patio or decking where you are going to be spending most of your time. This will create an unwanted glare in your eyes, which will stop you from being able to see the rest of the garden in comfort.
Think about sunken lights not pointing directly upwards. This is hard to do, but try to put them at the edges so the beam is not going to be in your eyes. This can happen with decking or patio lights which are centrally placed. It can be very off putting with a light shining in your eyes all the time. The only way to get the right placement is to experiment. You may not like what you have done the first time round, which is almost always the case.
The good thing is that most garden lights are very easy to move about and install, they are lightweight and very durable, so putting them in all kinds of unique places is not generally a problem. The only time you may struggle is if you have fitted the power cables to decking or wood. This is usual to hide these cables during the day, and to avoid tripping over them. In this case it is better to leave the final installation until after you have seen them in action at night. When you are happy, then this is the time for the complete installation.