Long Term Rewards
Tie an orange string (buy a ball of orange yarn at a store, it's only a couple of dollars and will last you a lifetime) around your wrist, and for every week you are SI-Free, put a white bead on the bracelet. If you happened to slip and want to count the slip, put an orange bead on the bracelet (either for each time you slipped or, for each week you completed SI-Free but had a slip or two, put on an orange bead instead). After a month, change the string to a different colour and put on a white bead for every two weeks you are SI-Free. Two months after the first month (meaning...in total...3 months free, sorry if I'm confusing), change it to purple, and put on a white bead for every three weeks you are SI-Free...
You can do any order of colours you want, or any way of beading it, that's just an example

You can stick with the orange, or change colours.
You can also do this with painting your nails, wearing earings, anklets, necklaces, rings, etc. It's a bit harder with males, sorry guys!
Create Your Own Rewards
In your favourite colour of ink, write/print off a list of positive things that make you happy, or your favourite things, and put them in a place you frequently see (i.e. your fridge, or taped on your bedroom door). In your second favourite colour of ink, write/print off a list of things you need to do, things you should do, and things you want to do and put it beside the first list. Each time you do something good (ie had a good day at school/work, cleaned your kitchen, ate a healthy breakfast, went for a walk, doodled instead of SIed, etc), put a check beside it (in your second list). Then chose a reward from the first list, a reward for something you did right, and use that reward to celebrate your fantastic feat!
Sorry if all this type is confusing to read, I'm a bit tired ^^