Choc's examples: Just say "honey, it would really help me if you could put the boys to bed tonite, I'm fried."
"I just can't get this whole house clean in time for your brother's visit. Do you think you could take two of the rooms, and between you and the kids, get them done for me?"
I have found that what works best for me and H is if I ask in a very direct, clear manner; the more words I use the more vague it sounds to him. Also if I ask in a way that implies he is doing me a favor, I then have negative feelings associated with the asking and it may set me up to feel guilty about asking him. Not only that, but it also makes me feel like I need to justify why I'm asking him to do something and that also sets me up for feelings of guilt and negativity about myself. I try to avoid saying "will you help me with...." Instead I say, "will you fix/clean/put away/etc.
In example 1, I would say: "Would you put S3 to bed tonight? Thanks!" I might add that S is all ready for bed. If not, I'd specify further if necessary: "Please help him with his pajamas and brush his teeth. Clean pajamas are in the closet. And here's a Pull-up."
In example 2, I might say: "I'd like to do all the house cleaning today because your brother will be visiting tomorrow. Could you please clean the bathroom this morning? Toilet, sink, and tub, vacuum the floor and change the towels. Thanks!" If there's another room I want him to clean, I give him the specifics on that room.
If I need something done right away I say so. Otherwise I give him a deadline (i.e., this morning or by 4:00).
Typically the way it used to happen here was that I would ask H to do a chore. No time frame, just "will you do x?" Then time goes by, H hasn't done it, I say, "are you going to do x?" H says yep. More time goes by, I'm starting to get annoyed, I say, "you said you would do x, why haven't you done it yet?" By now we're in full nag/avoid mode.
Recently this scenario played out; eventually H said that because I didn't tell him I needed it done urgently, he didn't think there was a big rush. So after that, I started giving him a time frame and no more problem. No more nagging, no more getting annoyed. Why it took us 21 years to get to this point, I don't know!