Have good conclusions slide: put there the main ideas, the ones you really want people to remember. Use only one "conclusions" slide.
The conclusion slide should be the last one. Do not put other slides after conclusions, as this will weaken its impact.
Having periodic "talk outline" slides (to show where you are in the talk) helps, especially for longer talks. At least one "talk outline" slide is very useful, usually after the introduction.
Don't count on the audience to remember any detail from one slide to another (like color-coding, applications you measure, etc.). If you need it remembered, re-state the information a second time.
Especially if you have to present many different things, try to build a unifying thread. The talk should be sequential in nature (i.e. no big conceptual leaps from one slide to the next).
Try to cut out as much as possible; less is better.
Help the audience understand where you are going. Often it's best to give them a high-level overview first, and then plunge into the details; then, while listening to the details they can relate to the high-level picture and understand where you are. This also helps them save important brainpower for later parts of the talk, which may be more important.