have you considered gamifying the process?
set up a system whereby people are able to amass points and / or badges by making suggestions. this can get complicated, as you can award further points for ideas being shared by other people, or for votes for and against, and also you need to minimise the ability to cheat, but, people like to feel they have been rewarded for sharing good content and they also like to feel they have earned something through merit
this powerful concept runs through many social websites, facebook likes, diggs, retweets etc. but this link is one of my favourites for demonstrating this power
another more simple solution would be to use suggestive graphics and buttons, for example, when a user signs in there could be a clear button called 'post your first idea' that would lead straight through to the form. this button could remain on screen next to a 'welcome' or 'you are logged in '. the button can then change, perhaps saying 'your last idea was liked by 5 people, why not post another?'. Another way to look at this would be to ask the question 'how visible is your form?' can people find it easily?
sites like linkedin do this: 'your profile is x% complete' then a link; the encouragement is always there to act
you might also want to have an activity stream too, so one person can see what their friends have done immediately on login and can click through to see the ideas posted