I do not have any experience using a DSLR taking portraits. I just know that around 85mm on 35mm format equivalent is ideal for taking full body portraits standing abut 5m away. If F/1.8 is used, then the DOF is about 0.36m (0.18m infront and 0.18m behind) and you get a very shallow DOF to have a blur background and the subject appears almost completely isolated.
Apparently, if you are using an APS-sized DSLR and want to maintain a similar shallow DOF, you would need to use a longer focal length and/or go closer and/or use a bigger aperture according to DOF. But then in order to maintain the same composition, you need to go further away instead of nearer. However, the longer focal length used to get a shallower DOF is negated by the further distance. (Use a DOF calculator to see what I mean).
For e.g. APS sized DSLR with a crop factor of 1.6x.
Focal length = 50mm
Distance necessary to mainain composition as 85mm on full frame= about 5m (about same as full frame because 50mm x 1.6 = 80mm on 35mm equivalent format)
Aperture = F/1.8
DOF = 0.67m (0.31m infront, 0.36 behind)
Focal length = 85mm
Distance necessary to mainain composition= 8m
Aperture = F/1.8
DOF = 0.59m (0.29m infront, 0.30 behind)
Focal length = 120mm
Distance necessary to mainain composition= 11m
Aperture = F/1.8
DOF = 0.56m (0.27m infront, 0.29 behind)
The decrease in DOF is very marginal because the shallower DOF of using a long focal length is offset by a need to move further away from the subject to maintain composition. So unless you can use a much bigger aperture, you won't be able to get a DOF as shallow as can be achieved on a full frame.
But then, I don't think you can find any very long telephoto lens with aperture bigger than F/2.8. Moreover, even a F/2.8 for long telephoto lens is very very heavy and expensive. That is why some serious portrait photographers still prefer full frame in order to get the shallow DOF which they can get with a relatively short focal length lens and a very large aperture.
So in the end, the practical way out for APS-sized sensor DSLRs is to have the subject stand further away from the background nad have a cleaner (i.e. not too cluttered background.