Arnica is a herbal remedy which is good for reducing bruising. You can get it as a cream or a tablet, though I think the cream is more common and easier to obtain from a chemist.

My father used to suggest rubbing the bruised area with butter. Not sure what the reason for that may be but a guess is that the rubbing moves the clotted blood away from the bruised area and therefore seems to reduce the bruising even though it actually does nothing.

Broken skin (you haven't said if you have any of that but you mentioned rashes) will heal quicker if it is kept clean - any infection retards the healing process. Essential oils like lavender and tea tree are good to add to bath water and have mild antiseptic effects. Be careful that you don't have an allergy to either first but these are usually safe to use for most people (unlike other oils which can be highly caustic). Up to 10 drops in a bath of either is a good volume to add to a typical bath. You can also use more conventional means to keep the area clean such as antiseptic wipes and covering with bandages/plasters.

Healing times for both bruises and grazes/cuts vary from person to person and if you bruise easily you may also heal slow. I would always advise a good diet to make sure you have enough nutrients for healing - things like antioxidants have been seen to be beneficial in surgical wound healing as they remove damaging superoxide radicals from immune cells (which are beneficial for removing infection but can also slow healing if in excess).

You could also ask (politely) if any Dom you work with is careful not to mark you too much and explain that you mark easily. There are techniques that can be used to reduce the risk of bruises - for example, placing a damp towel over the area when doing caning. One Pro Domme I know uses these techniques if the client requests it as some married clients do not want their wives to see the bruises. These techniques can also be used to reduce the risk of damage in someone who is very easily bruised.