I agree with what Annie has said, I use slip leads all the time for my two older dogs as they are both already trained to walk to heel but I have only started putting 7.5 month old Caoimhe in a slip lead in the last week or so. Prior to this I was still training her to walk to heel properly so I used a flat collar and normal lead, a slip lead would have choked her and could really have damaged her neck and throat had I used one when she was pulling. Teaching walking to heel is probably my least favourite training 'thing' with a pup, it is tedious as Annie says because it's just a matter of repetition and consistency and you don't get very far to begin with but if you stick at it it does pay off eventually. There are various (kind!) methods which work so it's a matter of finding the one you prefer and sticking at it for as long as it takes - the main mistake people make is that they try something for a period of time, usually about a week or two, and then decide it doesn't work when really they're just not giving it enough time.
It's taken me about 2 months of daily repetition to get Caoimhe walking reliably to heel but she now does it pretty much perfectly both on and off lead. I did it by first of all using treats to show her where I wanted her to walk and to get the 'heel' command introduced. I then stopped her pulling by taking walking backwards about 5 steps every time the lead tightened, I didn't turn around or yank her back at all, I just stepped back and she would follow me and once she was by my side again I said 'heel' and continued walking forwards. When she was walking nicely I praised her and reinforced the heel command. To begin with we spent more time walking backwards than forwards but we got there in the end and now she's great.
