How to squeeze a whole lemon so that as much of the juice as possible is extracted. And as a second priority none/not many of the seeds?  What alternatives do exists for properly squeezing lemons?  Differences to that question:  whole lemon/s, not just a slice, I know the trick with the fork no lemon crusher, I don't do that often enough to motivate the investment.  
Roll the lemon on a surface, squashing it slightly, before cutting it in halves. It will be easier to release the juice using hand pressure. Beyond that, accept that your hands will get covered in juice and press it until you can't get more juice out. You can apply more pressure by applying it to a small area and working around the lemon's circumference. If you want to avoid the seeds, you can remove some of them before you crush it. To be certain, squeeze it through a filter or into a container before adding it to your food. 