Here are some thoughts:
1) Don't take more players than you can handle. This divides the attention and gaming time, it also divides the overall fun for a game session. 4players is the usual maximum unless you're highly skilled or have a very simple united game.
2) Make sure your players have a chance to roleplay between themselves when you're about to be busy with someone else, perhaps not all players are in the fight, if so, try to make sure the others have some issue to debate or solve in the meantime.
3) Try different plot hooks: If your game is always about rescuing someone, it gets recursive. There are so many different hooks: Traitors, Quests, Rescue, Exploration, Mystery or Riddles, Diplomacy or Seduction, etc. Try something different.
4) Make sure your players have something to gain, if the only thing they can hope for is some treasure and exp at the end of the game, this makes it uninteresting. To solve this, try mixing elements of some player's backgrounds into the midsts of your campaign. Maybe you're exploring some ruins in a region close to where someone grew up, perhaps they can meet someone interesting and related to hang out with on the way, which could be useful or even harmful later on depending on the PCs reaction towards her...
5) Give your players proper reward. I'm always having trouble with this one

I tend to give a lot but to take even more from them haha. sometimes maybe you can try to stimulate them by giving them something unexpected, see what they do with it, and have it removed or better have them need to trade it for some higher purpose. That way your players will remember, and be talking about those "old days, when they used to wield this and that artifact(?)"