This works in 2 ways:
Things that you look forward to are things that are the same as, or similar to, things you've experienced in the past.
eg: returning to your favourite holiday destination or your favourite restaurant. With known territory, it's easier to anticipate what makes you happy and channel some mindfulness towards those aspects of the experience. Not to mention that the anticipation is half the fun (literally).
Projecting yourself forward into the future experience and then reflecting backwards.
Especially in the short-term (intra-day), it's easy to predict how you'll feel after completion or experience of some task. Having both the clarity of how you're going to get to some future desirable state, and the anticipation of success uplifts your current state as well. Imagine what it would be like to be excited about the steps taken in completing your tax return documents!
At the very least, forcing yourself into the future perspective is going to make the present more bearable. This is no excuse to fantasise of course, it's about visualising concrete steps taken to the desired state, and then projecting them back upon themselves. ie: It's about knowing how to get there, and that's usually all the incentive one needs.