When it comes to long exposure it really depends on what you are looking to achieve. I do this pretty regularly and in fact the primary reason for my having the P25+ is for this exact type of shooting since it isn't limited to a short maximum exposure, unlike my IQ160.
As mentioned, you want to use the T setting on your copal shutter. Any shutter release cable will work although if you are triggering your P25+ you'll either have something like a dedicated pre-trigger cable or a KG One Shot (or Alpa release).
For daytime shooting where I want to create some movement in the sky & make moving water really silky smooth I'll shoot for 1 - 10 minutes. These time frames will blur things like any traffic too but you can experiment to get the moving cloud look you're after. I tend to bracket, although using the exposure duration and filtration to get the look that I want. Once you get to 30-60 minutes you'll be making people and moving objects completely disappear in even busy urban environments.
I use either LEE filters (10 stop big stopper & 3 stop pro glass ND with IR block) or Tiffen IRND filters - 6 & 7 stop in 77/82mm or a combo with a B+W 10 stop ND.
The main things to watch out for light leaks/glare with the filters and heat build up/noise with the P25+.
If you use an iPhone then you might find a little free app called LongTime useful. I typically shoot a normal shot of my scene using the Alpa to establish the base exposure and then either use LongTime to tell me easily the amount of filtration needed for a certain exposure length or alternatively the total exposure needed for a particular filter amount. It's a lot easier than doubling exposures etc in your head (not that this is exactly hard either!).
This might be helpful regarding filters:
http://www.getdpi.com/forum/442289-post9707.html
Also, if you're shooting very long exposures you'll get through batteries. For night work I use a Canon CH-910 battery pack:
http://www.getdpi.com/forum/medium-...-kit-star-trail-photography-2.html#post482193