It helps to picture your client's arm in four sections- Front, Back and two sides.
First- Get your client to stand with her back towards you, with both arms stretched out behind her, hands closed into a fist.Spray from shoulder to wrist, pulling away into a V when you get to her wrist.This will cover one side.
Then turn her to her side, her hand closed into a fist, arm stretched out towards you, her palm facing downwards.You can place your hand over her closed fist, this will help you guide her arm(be careful not to grab her wrist as you can leave a nice set of white fingerprints :L.Spray from shoulder to wrist, again pulling away into a V when you reach the top of her hand.This covers the front.
From this position you can rotate her arm slightlyto the left and right, spraying each side.
Next, lift her arm into the air(so that her armpit is facing you), spray from her wrist to armpit. Working from wrist to armpit stops you from loading tan on the area around the armpit(i find a lot of clients have quite dry skin there from shaving, underwired bras etc and tan can gather there if you're not careful).Start spraying from slightly to the left/right of the wrist('Flick' your gun in horizontally then spray downwards in one move), and rather than spraying straight down the armpit pull away into a V. The overspray will give enough coverage to the armpit.
Finally, check that arms are done, get client to hold out her hands, palm down, and make a 'claw'(she'll feel silly but will have perfectly tanned fingers!!), give hands a QUICK LIGHT spray, then while she's still in a 'claw', get her to turn hands to left & right, so you can spray thumb & pinkie.
Hope this makes sense!!
Everyone has their own method, I think the most important thing is to come up with a step-by-step routine and then stick to it everytime, you'll never miss a bit!