Make sure your cover picture / photograph is of a high quality. It must also not be ‘deceptive, misleading or infringe on someone else’s copyright’. It is also against Facebook policy to encourage people to upload your cover photograph to their personal timelines. Your cover is what gets fans to your Page, so make it a good one. It also needs to express your brand and the ‘voice’ you have chosen for your brand.
Once you have uploaded it, test it on a mobile device to see what it looks like and make adjustments. Click on the cover photograph to open it up and add a description of what your Page is about? Add a link to your website and say something about the photograph. This could encourage people to like it and also interact with it, which will, again, increase your reach.