In my opinion you must use a foot retention system of some sort on a trike. It can be very dangerous not to do so - quite a number of novice riders have been severly injured by using ordinary pedals. The reason is simple - remember that on a trike your feet are out in front of you, about a foot above the road surface. If your feet disengage from the pedal at speed and a foot falls to the ground, it will be sucked underneath the rapidly moving trike. If you have very strong bones, the steel of your trike will likely be damaged. But if your bones are not stronger than steel, you may either crash or parts of your leg may be left behind you lying on the road. This has actually happened - so be forewarned.If you have heard stories about the dangers of clipless pedals - these are invariably from bicycle riders. Trikes are different.  Use clipless pedals if you are physically able to do so! 

I tried various clipless pedals. After considerable experimentation I settled on Look Keo pedals. These have been perfect - easy to get into and out of whilst at the same time holding the feet securely in place. They are adjustable with 9 degrees of movement - important for keeping the knees healthy. Feet on a trike must be able to rotate during each stroke or your knees will soon give out.

I have my pedals adjusted so that there is very little effort required to release (there is an adjustment screw for this). Good retention, good customer support, cleats easy to find, wide platform free from hot spots, eliminate foot numbness, and… they look good. I ordered cleat covers with my pedals which easily snap on when I am off the trike, allowing me to walk fairly easily with my walking sticks. I have also found that the Look Keo cleats last longer (and are less expensive) than Shimano cleats. Be careful when getting your cleats though - Look Keo cleats are different from normal Look cleats or the third party knock-offs from China. Get the red cleats (Look cleats are colour coded for degree of movement - red is best for trikes). And get the red cleats which have white teflon sections in them - these are pretty much obvivious to mud, ice, snow, or rain - all of which just rolls off the teflon.

I use Lake road shoes. Mountain bike shoes are IMHO far too flexible for trike riding. I like Lake's line for a number of reasons - they ship anywhere in the world, they often have sales, you can order online and not have to deal with "bike" stores, and they come in different widths. Although my feet are normal sized, I always order Lake's extra-wide shoe. Why? Because they can take winter socks without crowding the feet, and allow lots of foot movement (ie. cooling) in hot summers.

I also use short cranks. Normal crank length is 170mm to 175mm. I use 127mm. For me, this has meant far less strain on the knees, faster cadence, and less fatigue. If like me health issues mean that keeping a steady cadence is problematic, try short cranks. They have really helped me. Incidentally, if you use short cranks be sure to increase the distance of the pedals from your seat (i.e. increase boom length) by the same amount that you decrease the crank length. For example, if you shorten the cranks by 10mm, increase the boom length by 10mm. Very important.

Some Science concerning short cranks:

James Martin at the University of Utah Sports Medicine Center has done some interesting testing using cranks of different lengths. He looked at maximal sprint power less than four seconds duration using cranks of 120, 145, 170, 195, and 220mm. The finding? No significant difference. He also tested 145, 170 and 195mm cranks verses pedaling at 30-, 60-, and 90 percent of lactate threshold at rates 40, 60, 80 and 100 RPM. Result? Pedaling rate times crank length is the principal cause of variation in metabolic cost. Additional studies led him to conclude that one should cycle in a gear just below the highest at which one is able.

Now all of this was done on racing bicycles, not recumbents. And certainly not trikes. There are a host of other studies which I could quote here, but again, all done on upright (normal) bicycles. And always with a view toward maximizing power, speed, and to some extent, distance.

Trike riding is different. To maximize speed and distance traveled, I find that the combination of short cranks, moderate (rather than high) rpm, sufficient hydration and proper nutrition, and a gear one down (easier) than that which I feel capable of works best. Comfort makes a difference too - which for me means moderate suspension (an elastomer will do) combined with the right tyres and flexible seat mesh work well. I never pedal at my maximum power. For one thing, this is not really smart for someone like me with serious health problems. But more importantly, it is the average distance over time which I am interested in. A tired body cannot make this average distance high. It's a tortoise and hare thing  So while I am slower than my upright friends on their 5lb carbon fiber bikes, I tend to arrive at the destination more rested than they with far less need to rest along the way.