When getting checked for new glasses, how relaxed should you focus your eyes?

If you stare too intently, you partially compensate for your nearsightedness, and the glasses are too weak. I once did that, and the glasses were sharp but gave me headaches after about 15 minutes.

If you relax your eyes too much, the glasses are too strong.
Again, the view is sharp but start giving me headaches if I wear them too long.

How do I make sure I focus normal, not too hard or too lazy?

Don't spend a lot of time deciding. When they ask "which is better A or B, or 1or 2?" just pick quickly. Don't stare, wait, relax, or blur your eyes. It can take up to 2 weeks to get used to new glasses if it's a big change in your presription. Don't wait longer than a year or two between exams. If new glasses are bothering you ask them to reduce the rx a bit, if it was a big change. Also the way the glasses sit on your face makes a difference. The new frames should sit with the same distance between eye and lense, th same curve of the frame will help too. Have your optician adjust them to match your old (comfortable) pair. Have then adjusted once and awhile to keep them sitting right, so your looking through the lense correctly.

I worked as an ophthalmic assistant and did refractions all the time. (That is what you call the test for checking for eyeglasses.) You should relax your eyes. It is up to the person doing the test to make sure your eyes are relaxed by using certain techniques. If she gets conflicting answers, then dilation will relax them, and the test will be very accurate.