Well, a good concept is a good place to start for any type of entertainment medium. Of course, what is considered 'good' is usually dependant on the person writing the story. However, what necessarily works for that person won't necessarily work for an audience.
Other key elements would probably include: good props, decent actors, and of course...special effects.
Depends on the series- whether you want it to be comedy orientated (Sazer X, Go-Onger, Den-O), or serious, or right in between comedy and drama.
More arc or serialised storylines help, instead of constant monster of the week stories (though you can still have a decent arc and a different monster every week, depending how your story is planned).
Not having Toshiki Inoue writng it anime: joking!)
Anyways I think what make s a good toku show is:
-always keeping the story moving forward
-Not doing 5 straight eps. of crappy filler
-No plot holes! (I'm looking at you Kabuto )
-Monsters/enemies that look threatening instead of having retarded designs by using household appliances.