The thing about thought forms is they are energy.. Often emotional ones stick better, but there is always the the rule of use it or lose it, or out of sight out of mind. If you think about ghosts, you see the ones with unfinished business are usually powered by a strong emotion of sorts. (that also raises another issue, are there ghosts as well, or are all spirits thought-forms?( Trick question by the way)). That's what keeps them about, and also hard to get rid of. It is not easy or hard to keep a created thought-form, however. As we know, like seeks like, and so unused/unattended energy/forms will either become absorbed into a stronger one, or in some cases cleared away by similar means.
It may help to bind your thought-form to an object until it's presence is a common feeling to you. You could also tie it to time, ex: every hour, think of the form and re-establish a link with it, recall all the energy directly related to it. A while of doing this will help you to keep your forms in the front of the back of your mind, so to speak. Good luck, :)