Whilst the approach is an indvidual thing, too many spinners have run ups which are either too angled or too straight. If it is too angled then when the spinner arrives in the crease the tendency is to pull the front arm to the side diminishing rotation and affecting the amount of spin. If the approach is too straight the tendency is to jump in at the crease causing the delivery stride to be closed off. Again this causes the front arm to pull to the side or encourages the bowling arm to become too high, both lessening the degree of spin obtained.
Look at Cameron White and you will see what I mean.
Below are some tips which can successfully be used when assisting a spinner with his/her run up/approach.
Fit run up into delivery stride
Slightly angled approach
'Twitch' into side-on position one step prior to jump (bound)
Run up/approach is for rhythm and balance
Slight lean forward an advantage
Long strides better for rhythm than short steps
Focus on target at commencement of approach
Reverse chaining from the delivery stride, maintaining spin along the way, is a good method to discover the ideal approach.
If the spin stops, the run up is possibly too long, so move forward until spin returns.