You can abandon territories at the beginning of your turn, when you attack other territories. If you abandon all your territories, you can enter the map at a valid place (i.e., at the edge for most races/abilities). You can't abandon at the reinforcement stage at the end. Okay everyone probably said that already.
You don't receive special or racial abilities for your in-decline race, unless it specifically says so. Noticeably, the races/abilities that are effective when in decline have something on the back of the race/ability card. So when you turn them over when you're in decline, you can tell.
Also, if you have a race in decline, and you bring your active race in decline, then your in-decline race tokens stay there, but you no longer get anything for them. They basically become Lost Tribe tokens. (I replace them with Lost Tribe tokens when I play.) Nevermind.