I've done this twice now, once with the Amex Gold Rewards and more recently with the BA Amex, the latter requiring £3k spend in 3 months.

I was surprised how easy it was to do, but having learnt the process from the Gold Rewards I "held back" on some spend until my BA Amex application was successful. I then bought the full quota of BA Miles (taking advantage of the 20% extra offered at the time), got the car serviced, booked a holiday and bought a new suit. I then booked a few dinners with friends and rather than splitting the bill with two or more cards I encouraged them to either buy me dinner in the future or pay me their share in cash, whichever suited them best. In the end it took just two months and I didn't need to do anything involving buying vouchers to sell on, or anything beyond regular spend.

Of course all railway stations and all major petrol stations (should we call them diesel stations these days ??) take Amex. Throw in some supermarket shopping and dull stuff like buying homewares and it all adds up.