Ok, thanks for clarifying a little.
To answer your question, a good motto for ISTJ's is "Actions speak louder than words." I think that it would be a good idea to first directly talk to your ISTJ and let them know how you feel and what you're thinking (we can be horribly bad at reading people and guessing at their feelings).
Second piece of advice is to practice patience. The ISTJ will likely remain cold and untelling of their feelings, but trust me, a lot goes on under the surface that you never see. Beneath the cold exterior we are weighing pros and cons to infinity...this is where extreme consistency can only help you and where our rigid parts start to chip away. Don't put on a "show" of consistent behavior when you are around the ISTJ, you really actually have to be consistent across the many levels of your life (While spontaneity is lots of fun, unstable people are not). If you're telling me one thing, but news gets back to me that you have contradicted your word then I will not be inclined to believe a word you say. What I'm saying is you have to be REAL and honest with ISTJ's. It's totally ok to have faults/immaturity to work on, but be open and honest about them. Don't try to hide them from the ISTJ, because if they find out later that you act one way around them, and a different way around other people you will likely be viewed as hypocritical...which will make it impossible for you to have a deep realtionship with them (especially of the romantic kind).
Having patience is really important, because you have to allow us time to collect "data" and weigh the pros and cons (as in, you have to let us decide that you/the situation really has changed). Don't come jumping at us and asking for answers too quickly or too often. A little nudge now and again is ok, but don't be too aggressive. Find a way to ease yourself back into their life, make yourself visible and present, and then wait...let them come to you.
Really, consistency and time....that's all it takes. Also, noticing the small details will help you out too. ISTJ's don't look for big, flamboyant displays of affection, it's really in the small, thoughtful things that get us.