How can we minimize traffic loss after a domain name change?

HI Ngoc Le.,
I hope this answer is not too late.
You need to do following tasks in order keep the smallest reflect to your search engine ranking:
1. Keep your old domain alive
2. Keep your old domain and new domain parallel pointing to your website.
- You need to go to the cpanel and add on your new domain on exactly directory of your websites resources, remove your old domain or just ask the hosting provide to change your hosting domain if your are using that domain as primary domain.
- Park you old domain or using the 301 Redirects technique. This tells search engines and users that your site has permanently moved.
Check this video for tutorial on 301 Redirects technique:
http://www.youtube.com/watch?v=r1lVPrYoBkA&feature=player_embedded
3. Sign Up Google webmaster ( if you nare not sign up yet ) and tell Google about your change.
- Go to Site Configuration -> Change of Address --> Add and verify your new site to Webmaster Tools.
Follow the instruction to verify you sites then your are done :)
Recommend:
- Keep up using both domain names and using the 301 redirects to tell customers that you are moving in the new house.
- Tell customers about changes via a banner on the website
- Never give away your old domain.
Good luck :)