Look at the party composition before you start the encounter. As a GM you should fairly quickly start getting a handle on what synergies work (and hopefully who your tactical and creative genius players are) just look.
The first thing I would look at is class balance. 4E is strongly influenced by MMOs: you need tanks, healers, strikers and controllers. If you are missing strikers or controllers you're probably not too bad off. If you don't have tanks or healers you're probably going to have to make things easier as you have nothing but a bunch of glass cannons.
Also you can bring in environmental details to help the PCs--have them start making perception checks to notice advantageous situations--sun in the eyes, hidden gopher holes, what have you.
Reinforcements can go to either help your party or hinder them--or both. A wild bore might just decide the closest target (the NPCs) is the biggest threat and then go after the PCs (effectively moving some of the difficulty from the middle of the fight and tacking it onto the end).
Tactics are another one--if you aren't playing super smart NPCs--they can easily choose non-optimal targets, start to break and run away or any number of other personality defects causing them to be stupid. (Leaders stopping to speachify is a classic.)
And, if I understand what you mean by encounters (one off combats) sometimes winning is ok--the PCs then know they can loose.