Position
Good positioning is the key to success. Take the time to do it properly. (It can be slow especially in labouring women.)
The aim is to increase the space between spinous proceses by makingt the patient flex their back. Avoid rotation or lateral flexion to maximise your chance of being in the midline with your needle tip.
Decide whether you are going position the patient sitting or on their side. Sitting is generally easier but you should practice both positions.
In the sitting position instruct the patient to put their chin on their chest and to "arch their back like a cat." Most patients understand this. Alternatively you could put your hand on the patient's back and ask them to push out against your hand.
In the lateral position you should also bring the patient's knees towards their chest.