It's not so much about tools and tips, but rather taking smaller steps. Can you find an environment outside with fewer distractions? Or if you go out and there are more distractions, ask for easier, smaller training tasks?

If you can be more specific about what you are asking and what the distractions are, we can offer more help.

 

How old is your dog? How long are the training sessions? What are you asking him to do? What is distracting him?

 

Recognise factors that distract your dog so you can see which ones you can control in your environment.
For example, if it is all the smells and nature you could try to find an indoor space that is not your home to take the first baby step in telling the dog that listening is worthy everywhere.

Also, are you sure your dog really knows the behavior you have taught him and has not latched on the environment you started training it or to something YOU do only in that environment?
Another thing is whether the dog is or is not overexcited or scared of the environment.

I would pay attention to exercises where the dog seeks your attention instead of you trying to get his. He engages with you, he will be rewarded. He interacts with you, gets a task, succeeds, and is rewarded.

What are your rewards? The environment is a strong rival for his attention. Some dry, boring kibble won't do. Try something smelly and soft for rewarding.

Keep the training sessions super short. You can also use environment as a reward as in focus on me for this and then I'll release you to interact with the environment.

 

I've seen some use a whistle. Others show their dogs attention with a toy of his. Once command is done they're given a training treat. I hope this is of some help.