Saxon side-bends. Hold dumbbells over head, hands shoulder width apart, lean to the side keeping arms straight and dumbbells the same distance apart (do a search for a better description.) These will completely demoralize you and destroy your obliques.
Full contact twists - stick a Barbell in a corner load some weight on to top end and “Lets twist again, like we did last summer…” i think there are pics of it from a pavel article on this site
Rack the weight on one side, and walk for a good distance. At about 15 feet, you’ll REALLY start feeling your opposite oblique! Switch sides when you can’t take any more, and try and do equal distance with the other side.
Definitely like the tips from everyone else. Those Saxon side bends are sick. Another one you might try is a variation on a hanging knee raise. Instead of pulling your knees straight up, twist your knees up and really contract the obliques hard at the top. I usually alternate sides throughout a set, or you could just repeat on a side and then switch.