Yesterday, 23 July, I my Gmail account was hijacked. The perpetrators
had access to my account and as far as I know only one spam message
was sent. They had written a spam message in my vacation responder and
sent it out to everyone on my contacts list. I turned off the vacation
responder and changed my password but I'm worried that this might
happen again.

Here is their message:

Best supply!
Our shop is specialized in the export of brand electron products. We
offer grade  quality products ,Our products are authentic quality with
original box and accessories.
the products is including:  PC,TV,Camera,GPS,Mobile Phone ,MP3/4 and
so on.
Best Service,Highest Quality,Competitive Price,Timely Delivery as its
philosophy,We sincerely hope that we become your idea supplier .
Thanks for your visiting!
any further information needed is available if you contact us.
please touch website: www.btneeds.com
Email: [email address]
MSN: [email address]
skype: btneeds

Their website is for a company in Beijing China

How might I prevent this in the future?