[No is a complete sentence.][1]

> Sam: Could we continue executing on these steps?

> You: Sorry, Sam. I'd love to help more, but I just don't have the time right now.

This is true, still appropriately polite, but direct enough to establish your boundary.

If you have a hard time saying "no", make it a habit to never promise anything significant right away. Let's say, Sam asks you in person or on the phone. Then a conversation could go like this:

> Sam: Could we do this or that part of my work together?

> You: Thanks for asking. Let me check my schedule and get back to you  tomorrow at the latest.

Then check your schedule. Make a realistic assessment of *your* priorities and workload. Whenever convenient (don't let yourself be rushed), but at the latest on the next day, tell Sam what help, if any, he can expect when. If he needs more help, he needs to ask your boss to ease your workload in the meantime.

  [1]: https://www.goodreads.com/book/show/1266828._No_is_a_complete_sentence