Trimming: Big topic
... but here's some tips.
It's mostly a matter of your own personal tastes in how you want your tank to look overall and in sections (since you've set up islands).
Trimming aquatic plants like Vallisneria sp. is pretty straight forward - cut off the dying leaves only at the tips where it becomes damaged/brown/yellow. the young "plantlettes" will simply need to be separated once they have attained 3-4 leaves of their own by cutting the white runner between them and the mother plant. This runner trimming also works for Echinodoris sp. only difference is that you'll need to plant the young if planning to keep them.
Stem plants such as those in these videos: http://www.youtube.com/results?searc....1.8NCavxAkVmg
- when they reach more than your desired length cut off down to the bottom 4-5 inches or so, and either replant, or just leave the leaves for the uncut look and they will grow right across the top.(my personal favorite)
For carpets - let them grow out to where you want them then pull out any extra growth (give away or sell to store for trade if possible) occasionally hairgrass and such will need to be trimmed with scissors so they can regrow even fuller, since a lot of people need to see to understand... watch this video to get one way of doing this:
and lastly just watch everything online you can about aquarium plants, planting, and anything else you can get a hold of, plus read everything here, and the majority of people can and will help with things already posted.
You have a beautiful tank with a great start, and your here that counts for a lot!