Okay, let me first sort out some misconceptions about potassium and the heart.

Too much or too little potassium can cause rhythm disturbances of the heart. That much is true - BUT (and it's a very big but) if you have normal kidney function - and all LCers should have tests for this - any extra potassium ingested will simply be excreted.
As Gaijingal said, as we begin to low carb, the natural diuresis we experience can flush out potassium.
This level of low potassium can cause feelings of weakness and fatigue. You may feel that your limbs are too heavy to lift.
Your potassium levels on LC will not become so low that you will have heart problems. As others have said, we do get a fair bit from our food.

So, how much to take? The Drs Eades of Protein Power suggest around 400mg daily - in divided doses because it doesn't last in the body for more than 12 hours. You can either, as tofi suggests, take it in the form of No Salt (my personal choice) or take tablets. Take the tablets with food or they can cause tummy upsets.

And anyone taking diuretics or meds for high BP should check first with their pharmacist before taking OTC potassium supplements.

About the leg cramps: this is caused by a lack of magnesium rather than potassium.

We need around 400 to 600mg magnesium daily - another thing that can be "washed out" in that diuresis.
The best way to take magnesium is with calcium - they work best taken together in a 2 cal:1 mag combination. You can find them together in a 330mg cal:167mg mag combination. Again, the dose needs to be spread throughout the day.

HTH!

Rosebud