The old fashioned/ Field trial ways was not to train a spaniel to heel, until it is hunting like a mainiac, and even then most never really walk to heel. Esp the FT trainers are too worried it will impact on hunting drive. Se them on trials pulling handlers about no good for an average rough shooter or dog owner.
I generally attempt to get a dog walking to heel straight away when i get them at 3 months, then they don't know any better, althou with my GWp it was a real mission started geat got to about 6-12 months was a real PITA, fine on/off lead now thou but bloody hard work
Patience and repetition is the only ways, even get a lesson of a decent trainer is never a bad thing.
If ur timing for correction or when u say heel is off the dog could be understanding hee to be something different entirely. Same with treats or praise if ur timings off u could be rewarding the thing ur trying to stop.
Someone already mentioned the slip lead are u sure u've got it on the right way? Be amazed how many folk dont know, the metal ring should always be on the bottom part so comes slack straight away.
Try altering speeds of walk esp walking slow, dead slow changing direction and walking figure of 8's, but only praise and say 'heel' when ur dog is in the EXACT position u want, as soon as he pulls forward u must correct, no good letting it walk for 2,5, 10 mins out of position then correct dog doesnae know wot its getting corrected for. Just confuses them
I was always told a sharp jerk upwards in inwards is the best to corect as pulling straight baack just encourags it to pull.
Other things already mentioned like turning 180's away from dog (ie to right if dog on left side) as soon as dog moves in front, or stopping every time he goes in front. Often a dominance thing trying to be the leader of pack so out in front leading
if more persistant the figure of 8 with slip lead over nose works but have to be gentle as casn strain neck, or the FT hpr style put slip lead very very high on neck and use the stop to hoold it there sometimes tucking a lug into it (but sort of defeats the point of a slip lead, but most hpr pull a lot too)
If ur using a thin whippy stick to touch its nose u want the stick to be swinging like a pendulum all the time in front so dog walks into the stick, not u hitting it with it, give it a visual thing not to pull as always swinging in front of its nose.
Always worth gettin a lesson of a deent trainer, i had dogs for years afore i went to local gundog club and had a lesson or 2 with decent trainers, wot a difference it makes. It's so easy when folk show u the right/easy way.
Even reading books or on here or watching a dvd easy to misunderstand or miss an important but subtle thing there doing that makes all the diffrence.
A dog that is going to pull for the next 10+ years is a PITA a lesson is well worth it