StandardStretch WingsIf you're an experienced cup user:
Liners alone usually work great for menstrual cup backup. They come in a stretch wing style or with standard cotton wings. The stretch wings are trimmer and contain 3% lycra.

A Guitar Pick is a liner made for thong underwear.

The mini contoured backups worn alone work well too, but they don't have wings so you may want to safety pin them to your underwear to keep them from falling into the toilet. You will also want to wear snug-fitting underwear with this wingless option.

Sampler Sets

Super Toppers8 1/2" AIOIf the cup is new for you:
Sometimes it takes a while to get the hang of how to insert a menstrual cup so that it doesn't leak, and pads come in handy in case of accidents. I would use a light or medium pad to begin with for cup backup. A light pad could handle a day of dribbles, and a medium pad could handle one or two serious gushes or about four hours of light flow. Both of these pads are included in the Training Pads for Menstrual Cup Backup sampler set.

Recommended site: The Livejournal Menstrual Cups Community is a great source of information.