S.,
I see how hard you're trying! Here's a little motto I keep in mind: KISS = Keep It Simple Sweetie!
If you have trouble being consistent, you probably need a very simple reward program that will be easily managed.
I, too, am divorced and their dad is about 35 minles from me.
I need my girls (aged 6 and 4) to help around the house so I am not doing everything. I also want to teach them about responsibility and how to be somewhat orderly. I also want them to learn to take care of their belongings and respectfully live in our home.
Here's what I do. I bought a package of poker chips. Each chip is worth a dime. When they do what is expected (clear the table, wipe the table, get ready on time, etc) they get a poker chip or two or three. Sometimes I have a contest of who can get ready first (I always reward both because the worked so hard to get ready fast) or clean the room in a certain amount of time, etc. They have containers on the kitchen table where they keep their chips. Periodically we "cash in."
They dump their chips and count out the amount I owe them (great lesson in math). They give me the chips and I record how much money they've earned. I give them some for spending and the rest goes in a savings account (a great lesson in saving...when I can make it to the bank).
Note: In the beginning of this process, I gave chips for just about anything (including waking up with a smile). That got them very excited. When it seems boring, I'll step it up and reward for any good behavior I see (including helping each other, giving compliments, using manners, etc.)
For your purposes, you will probably have to "price" chores. I used to teach school and believe me, your children are not too old for this. EVERYONE loves to be rewarded!
Think about this for a few days then start writing down chores and how much they're worth. The poker chips can be worth a quarter for the older kids. You can post the chores and their value in a prominent place in the home. You want a constant visual reminder. Remember to add at the bottom that you will be giving rewards at your discresion for good behavior.
I would also add a part that "charges" for misbehavior. Not doing homework, 2 chips, hitting your sister, 3 chips, etc. Not grades, though. That's a separate issue.
You will hardly have to do anything! Your kids will be on top of it and will let you know how much they've earned. I'd be interested to know how it works out. It would work really well if Dad were on board and did it at his house but that's not necessary. Let me know if you have any more questions. Remember KISS!
ONE MORE THING I just thought of...The rewards part can be money but you can also make a chart that prices the rewards! 30 minutes of tv, one chip, 30 minutes on computer one chip, one hour alone time in room, free...etc. List whatever your kids like and they can "buy the rewards." You may want to have some things for free like everyone automatically gets 30 minutes of tv a day. Use it or loose it. If they want extra time, they have to buy it.
good luck!
S.