I have been researching this too as I have a low tech set up (a CO2 setup up costs over £125 for God's Sake).
Hardy plants that have been recommended to me are:
Java fern (Microsorum pteropus)
Java Moss (Vesicularia dubyana)
Amazon sword (Echinodorus bleheri)
Dwarf Anubias (Anubias barteri var. nana)
Tropical Hornwort or Coontail (Ceratophyllum submersum / Ceratophyllum demersum).
Carolina Fanwort (Cabomba caroliniana)
Water Wisteria (Hygrophilia difformis)
Red Leaf Repens (Ludwigia Repens)
Cryptocoryne wendtii
Dwarf Hyrophilia (Hygrophila polysperma)
Thai Onion Plant (Crinum thaianum)
Dwarf Sagittaria (Sagittaria subulata)
Beware that some of those plants grow far too quickly
At the moment I have in my tank:
Red Rubin Sword Plant / Rubin Medium (Echinodorus ‘Rubin’)
(note: not on the list & apparently 'will not last long in a low tech tank' - ho-hum, me & my impuses.
Green Cabomba (Cabomba caroliniana)
Amazon Sword (Echinodorus amazonicus)
Simple things I have learnt
I added some Laterite substate to my tank. Some one recommended it and after I put it down, someone dished it & suggested I get something else (at a cost of FIFTY QUID just for my small 15gal tank). I stuck with the Laterite but it was messy to put down to say the least!
Most plants come in a pot in some wierd green artificial mossy stuff. Remove the plants from the pots & (of you can) split them up (if you want) and try to remove as much as the green stuff to leave just the plant.
Once the plants are out of the pot, tie those that can come apart together with some thick cotton at the base of the plant. (The Cabomba were literally just cuttings, no roots, nothing).
I then just simply stuffed them into the gravel / Laterite substrate and then just added another layer of gravel.
To avoid introducing a new algae type to a planted tank with new plants, a simple bleach dip will work. The solution should be 1 part bleach in 19 parts water and dip the new plant in it for 1 to 2 minutes. The plant should be immediately rinsed the plant in running water to remove the bleach solution. After rinsing under running water the plant should be immersed in de-chlorinated water, which should neutralize any remaining bleach.
This will kill the alga spore but might, temporarily, slow down the growth of a healthy plant.
Note; If the plant is in poor condition it may die from this treatment.
My Future Planty Type Ambitions
To carpet some of the floor of the tank with Dwarf Sagittaria - kind of looks like bright green grass.
I have a lump of wood soaking outside (Mopani) onto which I hope to graft Dwarf Anubias and some Java Moss (using a hair-net apparently).
Oh and I might try a carpet of Riccia and hopefully get it to 'pearl' - but I don't think this is for a low tech tank.
Hope this helps (and more importantly is CORRECT,
Andy
PS
Having plants in the tank will also help to keep algae down as the plants will absorb the nutrients.