Sometimes children (say 3-5 years of age) want you to play with them, but for whatever reason, you cannot participate.
How do you say "not now" without hurting their feelings or making them feel rejected?
I found two ways of dealing with this.
First, when we get home I give my kids about 15 minutes of my time before attempting what I think 'needs' to get done. This way I have given them my attention and then I can say something like, I need to make dinner it will take about 10 minutes and then I will be back. this avoids the whole issue. Second, when the issue does come up I tell them about how long it will be and what we will do when that time comes, I then suggest what they can do while waiting.
The best thing is to let them know when. They won't mind "not now" if they know that their time will come.
We do this by using a timer. If I am working on something and the kids want some time that I don't have immediately, I will set a timer for 30 minutes, or 60 minutes and then when the timer goes off I make some time (even as little as 10-15 minutes). This way they know when their daddy time is coming and can see how long it will be. And when the timer goes off they can come get me.
Hope this gives you an idea.
I know how it goes... you're cooking dinner, or are in rush hour traffic, or on the phone arguing with your exwife (ahem) and your kid is all about "can I do bla bla bla?" while your brain is occupied and can barely eek out the cycles to say "not right now" let alone come up with a coherent answer.
My answer to your question is to change your own habit. Change-up into something just as low-cycle, but more meaningful... "ask me in a couple minutes" or "lets get home and then we can talk" . . .
Or, my personal favorite "Dude, i'm on the phone and you're interrupting. what are you supposed to do when I'm having a conversaton?" "Say 'excuse me dad'." "Right so let's try it again." . . . . "Excuse me, dad?" "Not right now."
So in addition to the other wonderful advice offered already. If it is just attention they want, it is often possible to involve your kids in what it is you need to do. Even if the request is, "Will you play with us Dad?" your answer can be, "Actually, I could use your help with something. Will you help me so I can get this done?"
For example, if you are getting dinner ready, why not have them help by measuring something out? "rinsing dishes" - yes, you'll probably have to do it again, but still - or just sitting and talking about their day while you listen and do whatever it is that needs doing.
With Laundry, I always asked my little one to fold washcloths and kitchen towels and to match the socks.
Even if I was creating a shopping list, I had (my own and sitting charges) "write" the list down for me just in case I lost my own list along the way. At three, you will just get scribbles, but that is okay because they can read it to you (and you made your own too). This often helped me because it prevented the request being made 100 times in ten minutes. Since I rarely used, "give me five minutes" or "you'll have to wait just a bit" they tended to be more respectful when they did have to wait (most of the time).
As usual, I love @balanced mama's answer, but to add:
Schedule special time once weekly with each of your children. Let them suggest several activities and you choose one from their list that you could agree to. Going to Starbucks, playing games, working on projects (a car, treehouse, etc) are examples. Even if its just an hour a week, your child knows they are a priority and that spending time with you is fun. This is a fantastic way to bond with your child and to keep it up through the years. The key is that it is something you both enjoy doing and that it provides opportunity for conversation (ie not movies). Both parent should do this with the child separately, and it should be a consistent, regular schedule.
It is far too easy for life to get in the way of quality time with each child so scheduling it really helps ensure it happens.
The younger your child when you start, the better!