I currently use Xamiol gel, and it's the best treatment I've found after more than 10 years of scalp psoriasis. I have posted in these forums before about it, so you could read those threads if you use the 'search' facility.
I saw a derm consultant recently, who'd cleared me completely using dthranol 5 years ago. When I worried about continued low-level use of Dovobet on both my body and on my scalp (but well within in manufacturer's recommended limits), he suggested I use it every day when necessary. With both body and scalp, I'm careful not to stop suddenly, but to wind down gradually to less frequent use.
It's betamethasone and calcipitriol i.e., Dovobet, in an oily base, instead of an ointment. I had previously found Betnovate and Dovonex scalp applications (in an isopropanol base) very drying and irritating. I also had good results with Clarelux foam (very potent steroid), but found that rather irritating and drying as well. The advantage of the oily base is that it softens scales and plaques, and makes any 'scalp muesli' easier to comb and wash out. The disadvantage is that it makes one's hair look greasy, but I'm past being vain.
Note that I find storing the bottle of Xamiol upside down is necessary, to make it easier to squeeze the thick oily gel through its small nozzle. At the end of a bottle I have to cut it open and use a finger to get out a large remaining fraction of the contents, which can't otherwise be used.