Keep a simple spreadsheet with the marketing strategies along the top and simply write each clients name in and tick where they found you. If it was a flyer, poster or FB post - which one? Gather as much detail as you can.
If spreadsheets aren't your thing, just use a A4 notebook instead. If you are in a busy retail environment, don't worry about the customers name, just pop a cross in the relevant box. Just find a way to keep track of it.
Be aware that lots of people will say 'on the internet' or 'on your website' so be prepared to dig a little deeper and ask either what they searched for or how they ended up on your website.
If you want a ready made spreadsheet to track your customers, give me a shout here and I'll ping one over to you.
PS As always, if you need help either pop along to one of our online Mastermind sessions or contact me for a 121.