For this to work you'll need to have downloaded "Dead Money".

1) At the Sierra Madre villa or casino make sure you find the holotape for returning pre-war casualwear to the vending machine. There are many scattered around the villa and casino.

2) Find any pre-war casualwear (and any other variant of pre-war clothing that adds +1 to agility) and bring it to the veniding machine.

3) Equip the pre-war clothes and select to turn it in to the vending machine. The machine will take your clothes but the +1 agility boost will remain, you can keep doing this as much as you please.

NOTES: Even if you completed "Dead Money" you can still do this, return to the abandoned bunker and turn in the clothes there.

Although the max agility that will show on your pip-boy is 10, if you keep doing this glitch you should still receive the benefits to sneak (and other stats) as well as total AP.