When I start teaching 'turn-arounds' (what spins are called around here), I exaggerate some of the positioning.
I want more arc or bend in the beginning and I want a horse very 'up in the bridle'. This is to keep a horse from dropping its inside shoulder and 'falling' toward the spin. It makes the 'turn-around' very limiting if a horse starts to lean in the direction it is going.
A turn-around is all about shoulder control. It is about a lot more 'push' and very little 'pull'.
I use a spur, but it is a 'press' of the spur and not a 'stick'. I use a 'smooch' simultaneously with the spur so then I can eventually stop using the spur at all and a smooch will add the 'hustle' I want without the tail swishing or the ears going back. In the beginning, I want exact form and do not care about speed for a long time -- a year or so.
A couple of other things to remember is to take your inside leg off of the horse when you ask it to start moving its shoulders. This is the 'signal' that most of my horses watch and 'feel' for.
The other thing that really helps is to ask a horse to go forward out of a turn-around. Say "Whoa!" and then immediately step forward if a horse even 'thinks' about stepping back. Never let a horse step back after stopping a spin. This stops most of that 'sucking back' thing that horses can get so hung up on.