Now what on earth is a slug jar you're probably asking. Well you can call it many things, slug jar, fat jar, exercise fund, mileage pot. Whatever you want.
The important bit of this is that it's a 'pat on the back' fund for you every time you exercise. So this means that once the jar is full then you get to spend it on yourself, for example: jewellery, book, cd, dvd, makeup, luxury smelly stuff, or if it's a big jar: expensive jewellery, mp3 player, massage, pampering day at a spa/health club etc etc. I'm sure you get the message.
I personally have a 'slug jar'. It used to house pickled onions so the money ends up smelling a bit dubious, but it's see-through so I can keep an eye on how the level is growing. Oh and most importantly I stuck the lid on with a lot of duct tape so it would be more trouble than it was worth to keep breaking into it to steal little bits.
So how does it work? What you do is decide on a monetary value for certain exercises. For example: £1 for swimming for 30 minutes non-stop. 50p per mile on your daily walk (as long as it's at least at 4 mph), 10p per 100 calories burnt on your heart rate monitor, 10p per 1000 steps on your pedometer, 10p per mile run, £2 a completed aerobics class etc.
You simply set a monetary value for particular exercises and watch the jar fill up.
Whatever you call your jar, it's a fabulous motivator, because the more exercise you do the more money you put in, the higher the level grows. Obviously I'm not advocating that you go mad here though. Don't turn into an exercise junkie just to fill your jar, because that'll have the opposite effect. It won't be a motivator, it will be a controller, and you don't want to end up like any of the sad over-exercised cardio junkies that you see around.

So go on, get yourself a slug jar, mine has £3.95 in it at the moment which equates to 39.5 miles run. I did think about also adding to it for the mileage cycled each week, but decided that running was the challenge that needed the extra little motivator.
Caution: As with all exercise, if you haven't exercised for some time, or if you have any medical conditions, you should see your physician first for a checkup. Tell them what you are planning to do so they can advise based on any medical history you might have.