Many authors will ask me when is a good time to start the cover process and start planing a design. My answer is to definitely get ahead of your writing and get a cover designed ASAP. The cover design will help you in a few ways that you might not realize. Getting the cover made will help to motivate you to keep writing and finishing this book. It will also help with planning out the design in the future. When the cover is made early it can help to strengthen the design later on by seeing things you might not think about or you may think of something different while you are writing.
When you get a cover done early I would suggest printing out the cover or even taking what you have written so far and sending it off to Amazon and getting a proof done. These bits of motivation can really help you visualize your book being finished and thus help with the whole process.
You also want to get ahead of your writing with marketing. If you are an independent author build up a following on the many social media sites and start talking about your book and posting the cover there. This will help build a following, pre-orders and hype for your content. You are also able to get early feedback that you might not have thought about while you were writing.
Always remember that it is best to do as much as you can before you publish or release the book, once the book is out it can behard to change things, so get as much out as possible, get feedback and then make edits and changes based on the feedback you hear from your readers and fans as they are ultimatly the folks who are going to buy your book.