Personally, I don't remember Kiddo's constant requests for help for this project or that, so I have a habit of writing down those requests.
What's more, if you keep a list of the requests, you can start to 'plug them in' to your day's plan.
Most mornings, Kiddo and I have a ritual. We sit down with a piece of paper and make three columns: need to do, want to do, plan for the day.
Need to do: I write down every.single.thing which needs to be done, including meals, my shower, dishes, etc. and any appointments and Quiet Play Time, so he knows it's important.
Want to do: self-explanatory, and this is where I ask Kiddo to choose some things from that all-important project/activity list
Plan for the day: Plug it all in, in relative order. This way, Kiddo knows that "after mom makes morning snack, she's going to help me cut up that cardboard box before she does the dishes". It eliminates a lot of nagging and I am able to get my work done.
For longer projects, too, carve out half-hour chunks and explain it to her as such-- that a scrapbook is an ongoing project, and that you can help her at X time today/tomorrow, etc. My son is always making something or wanting my help. This keeps us both on track with each other and gives the day structure. And if I do have a minute, I'm happy to help immediately.