Filters on the lens, filters over your lights, filter effects in PS and Warm Tone paper. I don't know how widely spread warm tone paper is for digital but a girl at uni just found a box in 'Jessops' and it's really lovely, honestly.
The effect you use all depends on how natural you want the warmth to look.
Like a lot of things in PS, there are many ways to do this. All of the ones suggested earlier work, and you could also go into curves and alter the R, G and B curves separately to introduce the amount of color you want in the tonal range you want. Something similar can be done in a color balance adjustment layer. This is all assuming that you've converted to bw using some sort of adjustment layer so that you still have a rgb image, not a grayscale image. (except for duo- tri- quad-tones, where as far as I know you have to go through grayscale first) You can also tone during a bw conversion through the adjustment layer, at least in PS5.
thanks. I'm not too good with photoshop so I will just try using a warming filter for now. Might start playing with curves when I start to become more comfortable.