Dust mask, hair head scarf type thing, and some kind of overall or in my case a revolting coloured house overcoat donated by granny that is totally hair proof and will also withstand a nuclear exposion as its some kind of "supped" up nylon.
Daylight is always good, and a radio is a must.
I will then have rags on stand by to remove excess hair and I use a few drops of oil in water to get rid of the finer chopped up hairs.
Make sure coat is really clean.
Make sure blades are sharp or else they'll pull and hurt, don't continue if they are.
Check clippers regularly and brush off before oiling regularly.
Make sure the lighting and flooring is good and safe.
Make sure someone knows what you're doing and checks regularly (a friend got kicked into next week, why they have still not fathomed; she was very experienced head girl, horse was old hunter clipped every fortnight). She was slammed against a wall, collapsed and nearly died through lack of oxygen if someone had not heard the thud and investigated. That triggered off MS; she suffers very badly but she is very lucky to be here at all. OK, a very freak accident but a warning all the same.
Probably will be frowned on, I don't know how professional you are, but be ready to twitch, don't be mauled about. If you do twitch, be quick, don't leave it on too long even if you have to go back to it again; it's not fair on the horse, 20 minutes should be absolute max'.
The person helping you should always stay on the same side as you so horse doesn't move into you and be quick thinking enough too.
Elastic bands over your wrists helps keep a lot of hairs out and I personally don't wear a bra when clipping else you find hairs in it for months!!
Do the face/head first, before the blades get warm. My last horse used to get really upset if I clipped her face once the blades were at all warm so used to do the head first.
Buy, beg or steal an old all in one motorcycle water proof suit - wear wellies and rubber gloves - you are then sealed top to toe and you don't end up full of itchy hair.
Sharp blades, clean horse, draw out lines if you can't do it by eye.
If horse is not 100% to do, have someone with you and don't take risks - if it won't stay still with twitch, food etc then have the vet out to dope it.
Keep checking the blades are not getting hot and clean and oil as you go.
Don't have a clydesdale - they take forever as they have brillo pad hair and are huge
I have today spent 5 hours clipping 3 horses today, one had everything off and the other two full apart from legs. I have found over the years if you get a small tub, (supplement one) I use a blade wash and put all the blade wash in the tub and then dip the blades in the wash in the tub while they are still running, and then oil them over the tub, this cools the blades and also cleans them thoroughly and preserves the blades. Bandage the tail up to keep it out the way and I normally put the mane in rubber bands as well and make sure you have a sturdy box to stand on for clipping faces and up the mane and make sure you have plenty of tea to drink as its thirsty work.