1)
Work on it consistently. Ideally, work on it every day. It doesn't have to be a big block of 2 hours each day, browsing through bug reports for 10 minutes can also be enough. But it has to be consistent.
The benefits are:
* It keeps the project fresh in your mind and ready to go. No time wasted on figured out where you were at the start of each session.
* It avoids the that demotivating feeling of guilt that may creep in when you think that you've neglected your project, or once again "started something, but not finished".
* It provides a consistent sense or forward progress which helps generate motivation.
2)
"Motivation" is a resource which needs to be managed. Projects will have a mix of interesting tasks (motivation generating) and dull tasks (motivation consuming) tasks. Be aware of this, and mix these two types of tasks so that you don't bottom out on motivation. Developing some self-knowledge of how you work as a person pays off here too.
Demotivating tasks can also be chipped away at by working on them a little bit per day over a longer period of time. Consistency is the key here too!