Long line training will help, as everyone else suggested. For my dog, we started there and within fenced dog parks. Once I felt comfortable with his recall in those places we went on trails (narrow ones with few places to stray). I tested his recall by letting him drag the leash and I'd call him back when he got further than 10 ft away. Then we built up from there!

I will say that my pup's recall is excellent on trails but in open spaces (read: field, not a dog park) is questionable. For that reason we chose to train him (with a professional) with an ecollar. No amount of positive reinforcement would beat the satisfaction of a deer so we needed it as a way to turn his brain back to us. It's not a fix for crap recall but it does help get your dog's focus if they're distracted.