I tend to make an alpha channel by cloning one of the colour channels ( drag one onto the small icon at the foot of the layers pallette ) - usually the darkest one - and then adjusting it from there.
The brighter the alpha channel is, the more ambient light colour gets added to the texture colour, so the darker you need to make the texture. Don't try using real colours, they will nearly always turn out too bright; as harry said, taking 25% off the brightness is a good rule of thumb. If you do it as an adjustment layer on top of everything that will make your white text a bit grey ( which is sometimes what you want anyway ), so usually I adjust the texture colours or use an adjustment layer as a clipping mask if I can.