The above posters pretty much nailed it.
If you have the fridge space, just plate them up ahead of time and be really fussy about how you arrange the ingredients so they look nice. Fan stuff, shingle stuff, stack stuff. I assume you're using something like a vinaigrette to dress them? Just get it good and emulsified and leave it in a squirt bottle. If you have white space around it on the plate you can just give it a couple go rounds with the vinaigrette, assuming the plate's flat. Two color's would be cool if you had time. Like an orange sauce, thickened to the consistency of the original dressing, but it sounds like that may be a stretch. If you know someone who garden's they're probably cutting off the basil and oregano flowers from their plants right now, and a lot of people throw them away. Free garnish that looks nice, tastes good, and is mild enough to go with a salad.
Chicken: depends on the stuffing and depends on if you're cutting it before serving or leaving the breast whole. Asparagus has been used as a garnish before. Depending on how you're doing the potatoes, I would consider layering it, especially if the onion sauce is something you'd want on the taters. Slicing the cooked breasts will make this a lot easier to do, so the guest can just stab it with a fork, instead of trying to cut a chicken breast on top of a soft piece of potato. If it were me, I'd probably set the sliced breast on the potatoes, sauce it, run a line of sauce around the plate, top the breast with asparagus spears, and then set oblique cut carrots on their ends in the line of sauce around the plate. Maybe sprinkle on an appropriate chopped herb, or put a little ball of caramelized onions, fried onions, or thinly sliced fresh onion on top.
With salmon, your choices should be affected by how you season it. I'm a fan of fennel tops, butter sauces, dill, deep-fried spinach, and I'm sure a lot of other things I'm not thinking of at the moment. I would avoid laying the fillet flat. It usually looks boring, but it all depends. I'm not a fan of lemon. To me, it's like the chef is saying, "I don't know if I seasoned the fish well enough so here's a lemon to squeeze on it just in case." If it's not something squeezable, it seems like garnish for the sake of garnish. Same rules as above apply. Fish that is baked until flaky (I assume that you'll be doing that) stacks well. since it breaks apart easily.
In general, pay attention to how the veggies are cut. Little things like striping the cucumbers or decorative cuts with the carrots will go a long way toward the ultimate look of the dishes, and since they are done before the service begins, they don't make life any more hectic than it already is. Good luck.