Set up your encounters like you would visualize the menu at a restaurant. Just like a menu groups types of food and related dishes together, do the same for encounters, using a theme, environment, or circumstance as the guide for grouping encounters. Concentrate on role playing the encounter than on the numbers involved and use game rhythm as your pace on the when and where to have encounters. Action is more fun than scenery; don’t’ bore your players with a description of how dangerous, twisted or wonderful your latest something is, involve them in it to discover that fact firsthand. People and personalities are more interesting than any amount of landscape, dungeon scenery or beasts you can describe. Use that to your advantage to intrigue and entice players along a path of your creation while keeping them entertained with the idea that they conceived the choice to take that path.
Don’t give suggestions but woo players with concepts and ideas: form intelligent, intriguing plots and have them in action whether they players participate or not. This lends to build the background story of your world and adds realism by the bucketful. You can then use this as a tool to build diversity into encounters. Your antagonists may act according to this (known or unknown) plot line, acting intelligently some of the time and then foolishly another, at least from the players perspective. However, as a word of caution, don’t make too many diverse plot lines; it can frustrate your players. The easiest way to handle this is not telling your players everything. Also, don’t always tell them the truth or lies. Make them work to find out the details but only to a point. When you begin to sacrifice fun for detail, accuracy or realism, pull back some and rediscover that balance. Above all, remember, keep it simple but make it fun.