Vested interest - a personal reason for involvement in an undertaking or situation, especially an expectation of financial or other gain. Example - "Banks have a vested interest in the growth of their customers"
To press/push one's luck - to try too hard to get a particular result and risk losing what you have achieved. Example - "She's agreed to help on Saturday, but I think I'd be pushing my luck if I asked her to be here the whole weekend"
To end something on a high note - to finish, complete or leave (something) at a successful, impressive or climactic point. Example - "I know we've had quite a few losses this season, but let's win this game and end on a high note"
Pecking order - the way in which people or things in a group or organization are placed in a series of levels with different importance or status, a social hierarchy. Example - "As an assistant manager, he was pretty low in the company's pecking order."