I have a selection of answers for this, as I am an eldest and was always being asked or assumed that I could take my sister with me, or do this and that on the way to something else. Then I would get very uptight as I hate being late for something I had promised to do. So here is a selection of ideas , some of which you might find worth trying, and I do think that when you are beginning to try different ways, it can be good to say out loud on your own the various comments or sentences you might want to use and then you can also get the tone of voice right too. We can say exactly the same sentence and yet get a different response depending on the tone of voice or the way we look when we say it. Not letting your voice rise, or sound angry but just normal, so that you sound as though you think this is very reasonable about the situation and expect no come back from them
So, an immediate reply ;- I am already doing something that day (this does not say sorry nor does it mean that you have to declare what this is. ) It could just be you want a time on your own or whatever.
Then ;- I'm not free that day, but I could help a bit on friday. - so you have offered help on your terms and clearly said you are not available on the original day. If you want to you could always go back and say that such and such was cancelled and you now could help on the original day
Another is ;- I am overcommitted at the moment with family things that I cannot change so unable to help this time.
Or ;- I havent been well and have got very far behind with many things including tax bills paperwork or whatever and am not free to offer any help until I get things under control.
Then ;- I am expecting to go to my (daughters/mothers/friends ) in .... days time and have too much to do to get ready.
If they continue to badger you, dont allow that to make you renage on your intentions. You could always get together with another friend who has similar problems and be there for each other . So in simple terms you just say I am out that day with Mary, or we plan to meet in York that day. Now that is the word PLAN. Stick to your guns and if she sees you on that day and you are not in York you simply say it was a shame as something came up and you have rearranged it for another day. Then that day can be whatever day you are asked for more help!!
So , if you are really fed up with one particular person constantly asking your help but never offering you any in return, you keep this up until the penny drops and she either starts to give as well as take or she just goes elsewhere and you dont miss her.
We are constantly expected - especially by men - to be superwomen who can work and pick up their glasses or look for something that they are perfectly capable of finding for themselves. So practise saying in a very even voice, Im very busy havent time to look for it. or the more usual "have you tried looking in the car" but whatever you do, do NOT go and look yourself. When they half look and then come whining I cant see it , then tell them the torch is in the left hand drawer and try again and while they are there please look for your glove while they are there. If they have the cheek to say they havent time to look for your things etc, the simple usual reply could always be WhY? what did your last slave die of? . If you wish to be less confrontional, another rather pleasing things is to listen to their woes on losing something, and commiserate, Oh thats a shame, but when they look expectantly for you to stop what you are doing, just carry on with your own task. That way, you have not refused to help , but you show that you do not see it is your job to sort their problems out. Or of course that one where you suggest 2 or 3 different things that you might suggest but under no circumstance do you stop what you are doing and just continue . When they moan again, you might say, Oh dear, you may have to go and buy another one. You are acknowledging what they are saying , but making it very clear that it is not your problem and you have no intention of doing anything about it. You need to keep this up for some time until they stop using you as a doormat and then on the odd occasion you feel like helping or you might have a good idea where something is you can look and then they should be more thankful, or alternatively set a time limit, so you say I have five minutes to spare but have to leave/cook/watch my programme/go out. Good luck, enjoy having a go at the different ideas, see which ones you enjoy doing and which seem to work well. Keep a little notebook privately and make a note about what you did, how you felt , and how well it went. Keep it private and then in a couple of months you can look back and see if or when you started to get different reactions. Let me know how you get on. It will be interesting to see which has the best affect. You will get some pleasant little laughs at the reaction when they get a total different answer to that they expected.