Alternate shapes and sizes. If she can't yet hold her own sippy cup, try different types -- with handles or without, wide or narrow -- and gauge her comfort. It could also be too heavy, so cut back on the amount of liquid in the cup and see if that helps.
Challenge her. Don't rush to assist right away if she's having trouble. Give her a chance to try first. When it seems like she really needs your support, provide the minimum help possible, such as propping up a difficult-to-grab toy rather than just handing it to her.
Offer a reward. Give her tempting objects, like Cheerios or a pair of colorful maracas, to practice picking up. She'll have more incentive to try when there's a treat for her efforts at the end.