Make sure to begin combos with feints, and end them with footwork or head movement, and from here, to practice them as such. This means turning a simple 1-2 into a 4 or 5 move combination with preliminary set-up techniques, and post-liminary defensive techniques. For example, feint body jab or cross, jab to head, side-step with rear foot, pivot off rear foot. An alternative would be to pivot off the lead foot after jabbing or throwing a right hand, and also, to hook while pivoting off the lead foot (this is a simply awesome technique, but will take some practice especially if you are looking to do it immediately after jabbing, which, while challenging, is very possible for anyone and is well worth it).
Beginning a combo with a feint or fake (a fully thrown strike which you have no intention of landing; jabs work well) is just good practice, and side-stepping or pivoting (ideally both) after your combinations will force opponents looking to clinch, duck under or shoot on you to first turn and close the distance and make you much harder to take down. Also, it is important to vary feinted strikes with faked ones, as well as body feints performed using the shoulders, hips, knees, chest, and eyes. This will induce uncertainty into your opponent, creating opportunities to land strikes, and, at an advanced level of application, allowing you to control his reactions.