Normally in RL we hate hat hair or helmet head... but in SL, this is the time you really want to have a few hat hair styles hanging around! Prim hair just doesn't behave like real hair and sticks out. The trick is not to get the hat to fit the hair but the hair to fit the hat! Just follow my instructions but first MAKE A COPY of the original hair.
TEN STEPS ON HOW TO MAKE HAT HAIR
1. Make a copy of your hair and rename by adding Hat Hair to the name of the hair
2. Wear the hat hair copy and a desired hat - preferably one of mine :)
3. Stand on a posing stand and right click on hair, choose edit
4. Check box - Edit linked parts
5. Check box – Stretch. * Make sure Stretch Both Sides is NOT checked
6. Right click individual prim hair
7. Decrease size of prim using the appropriate x, y or z until it disappears inside the hat
8. Repeat for all hair sticking outside of hat
9. If you make a mistake on an individual prim, right click the prim and in the Edit Menu above the screen, click Undo.
10. Be patient since it may take a few tries to get it right. It is really worth it to have that perfect look when wearing a hat.