from Heathrow Airport:
If you're flying into London via Heathrow airport, and want to make your way to St Pancras, you can simply take The Piccadilly line on the Tube, London's public subway system. Departures from the terminal are frequent, and it will bring you to King's Cross St Pancras station in just under an hour.
You could easily catch a cab, though the cost is considerably higher and the arrival time will be similar. However, at peak times the Tube can be very crowded and hard to maneuver with luggage.
Another option is to take the Heathrow Express train from your terminal to Paddington Station. This costs 15 GBP (about $30 USD) and takes just 25 minutes from the terminal. Departures are frequent and the trip is very comfortable. From Paddington, you can hop on the Tube's Circle line, which will bring you directly to St Pancras.

from Gatwick Airport:
If you're arriving at London Gatwick airport and want to make your way to St Pancras station to catch the Eurostar, there is a train called the Gatwick Express that will bring you to Victoria Station in London in about 20 minutes or so. You can easiliy make your way to St Pancras from there by taking the Victoria line on the Tube.
Alternately, Thameslink trains run from Gatwick every 15 minutes throughout the day. The Journey time to King's Cross is 45 minutes and it costs about £10 (about $20 USD) for a one-way ticket.
Taking a taxi cab is of course another quick, though more expensive, alternative. The 45 minute trip will probably price well over $100 USD.

By Tube
The underground station Kings Cross St Pancras leads directly into St Pancras International and is better connected than any other London tube stop with 6 of the major tube lines running to it; Victoria, Hammersmith and City, Piccadilly, Circle Metropolitan line and Northern.
Check back for updates as they become available. If you have any questions about traveling on the Eurostar from the new St Pancras station, please don't hesitate to contact us- we're happy to help!
St Pancras