http://docs.opendatakit.org/collect-location/ has good documentation on how you can improve location performance. In general, location is determined by the GPS chip in your device and a having a clear view of the sky.
If you are having problems with Collect's location, it's also a good idea to use a tool like GPS Status to confirm that you are seeing as many satellites as possible.
We'll be improving the geo user interface in Collect soon by showing users more information about what the GPS is doing. Follow the topic at Show satellites and time elapsed in geopoint dialog to get more information.