My suggestion would be to get her a walker with or without wheels, but if there are wheels, make sure they can be locked. Let her lean forward and pull herself up while you steady her. This is also the best way of geting up from the toilet and out of bed.

Sometimes you can pick up a cheap walker at a thift store or Good Will.

If you are somewhere and the walker isn't available. Stand in front of her, brace yourself and encourage her to pull herself up using your arms for something to pull against. She needs to use HER muscles as much as possible. when you try to do the lifting, it sort of works against her body's way of working.

I hope that makes some sort of sense.

The advice Agnes gave you about using a walker is really good. Also, your mom might have more trouble if the chairs are too low or too squishy. You might want to invest in some pillows (such as those used with dining room/kitchen chairs)to raise her up a bit.

You also might want to visit a special needs store to see some of the gadgets available. There are special lift chairs too, but they can run close to a thousand dollars or more.

One last thought: You need to be careful about how you "help" her or you could end up injuring yourself.

IMO the upper arm is a much more useful place to hold than anywhere else in this scenario.
Face her while she is sitting. With your right hand grab hold of her right upper arm, and she in turn will grab hold of your upper arm with her right hand. This makes for a much more stable assist than relying on grip strength, or trying to lift her whole body. It allows you to support her pulling herself up with your entire body and not just one arm.

Difficult to explain, but try it.

Cearbhaill has described that way of allowing her to use her own muscles much better than I did. It's important that she keep that upper body strength as good as possible.

If all her chairs at home have soft seats, try this. Some folks will put a piece of plywood between the cushion and the springs. It keeps the body from sinking down so much.

Thank you for the replies. I'm afraid I didn't give enough detail about the situation but I think Cearbhaill probably has the answer. My mom was given a 6 month to live prognosis about 4 months ago. About two months ago we moved her into an assisted living facility...didn't want to go, still felt ok at the time. It was a good move for several reasons. She has no trouble with the furniture in her room. She does have the trick walker with wheels, seat, and brakes. Works well for her. The problem is with the chairs in the cafeteria, and that's just been over the last week or so. They'll bring her meals to her room so that's not a big deal as far as getting something to eat. Sometimes friends or family drops by and she would like them to share a meal with her at the cafeteria. She's getting to be afraid to go since a few times she couldn't get up from the chairs there. She's always liked having meals with people and I'd like her to continue doing so as long as possible. Thanks for the suggestions. I'll give it a try next time.

Have the nurse or aid show you how to do it? Or maybe a lightweight wheel chair to set at the table. She could use this only at meal time. They do it here at the homes.