My dog will not let me pick her up?

I've had my dog Holly for a year now. I rescued her from the pound. She's a sweet dog, cuddles with me on the couch, when I go to bed whatever. But when I try to pick her up she runs around playfully and won't let me pick her up. Anyone have any ideas how I can gain her trust into letting me hold her? She's damn fast and she's gotta go to the vet soon. Also she's part weenie dog part beagle. About 16lbs