I have two 7mth old blue merle dogs and the one is a total nightmare to walk on lead,i have bought both a harness but im still having the trouble of the one constantly pulling hes on a short lead but the strain of holding him when walking is terrible he dosent want to walk but run,when we are out and he goes to pull of i stop trying to let him know that im the boss but im not having any luck whats so ever he just gets it in to his head that hes got to pull please any advice would be grateful

http://www.learningaboutdogs.com/acatalog/Specialised_Clicker_Training.html

Walk with me Book is great, Kay also does regular courses on this.

Regular harnesses (I assume that's what you're using?) don't do anything for pulling - if anything, they can make it worse, although personally I find the dog a little easier to hold on to - no risk of collar slipping.  One harness that I think is a good one for pulling is the new Halti harness - it works on the same principles as a regular halti, you clip the lead to a ring in the centre of the chest strap, and when the dog pulls, it gets turned to the side.  So same idea, but no risk of neck injury to the big-time puller.

One thing I would definitely do is walk them separately - for one thing, it'll make it easier to train the puller, because when you are stopping to correct his pulling, you are stopping and correcting the other for walking nicely - it's a no-win situation, really.  Plus, they could be egging each other on - I know my two walk better individually.  It is a lot of work, but well worth it - as someone said recently to me, you get out what you put into a dog.  And as he's still young, you have a good chance of sorting it out if you really go all out with the training.

Check out this link:
http://clickersolutions.com/articles/index.htm#behaviors

there's some good stuff on there about loose-leash training - one article I liked was about traget training with an australian shepherd, that was still allowed to pull outside of trainnig sessions, and it worked.  Good luck!