Negative Declined Twist
Working most forms of abdominal exercises in a negative return is an excellent way of developing muscular strength, as the muscles are placed under contraction for generally a longer period.
Initiate the movement, beginning at a higher level for what would be your final phase, if performing the exercise in a normal manner.
For example, if performing a normal oblique sit-up, your final phase would be lifting your upper-body 30 degrees off the floor. In a negative format, aim to start with your upper-body in a position of 45 - 60 degrees off the floor; you may want to use your hands to achieve this position.
From a 45 - 60 degree angle, lower your body down to the floor, in a slow controlled manner, aiming to take as long as possible to bring your body down to a normal sit-up starting position.
The length of time that you can take will obviously determine the difficulty of the exercise, beginners should aim for 3 - 5 seconds, intermediates 5 - 8 seconds with advanced being 8 -10+ seconds.
Repeat for a desired number of reps that work your abdominal muscles, whilst keeping good technique throughout.
You will find that performing this method using a variety of style of abdominal exercises, will require you to adjust the time scale of your negative phase, the harder the exercise, the reduced time required.
Aim to work one rep at a time, in slow mode, rather than adopting the negative position straight away to perform the next rep. Concentrate on feeling the abdominal muscles working in a slow contracted action.