1. What team are you playing ? If you are playing a highly ambitious team or a professional one, then the problem should be solved easily. First gain their trust, go to press conferences and always back up your team. If at a press conference you're asked what expectations do you have if you're next match is agains a low club say that the match won't be easy and the team must work hard to achieve victory.
2. If you play as a beloved team (Arsenal, Barcelona ...) with lot of fans, in the pre-match team-talk say "For the fans" and if you're leading at half time say "Do not get complacement" This combination of team talks always worked best for me when complacency has been an issue.
3. If you have good player in your squad (AMC, ST, MC) and he ruins a lot of chances, but in one match he scores, DO NOT PRAISE HIM, as he will feel that scoring a goal in 10 games is enough, which in fact it isn't. Use this tehnique with players that should score goals, not with defenders.
4. If you have done everything I said and they still get complacent, then you should take more radical measures. Take a look at your captain and vice-captain. A captain's main job is to drive the team forward in the game, but if himself becomes complacent, then you should replace him. Name another player as captain.
5. After every match ask your assistent manager (if you use one) to give you the Team Talk Report. This way you will be able to see how and which players reacted to your team-talk.
6. If none of this is working, then you should look at your tactics, maybe your players want, but just can't.
Hope I was helpful. Cheers