Things to make online transactions safe (in no particular order):
1. If you are using Internet banking, enable authentication via sms on each transaction
2. Always verify the site is secure (HTTPS at the beginning of the URL)
3. If you are using credit card, see if your company has an option for disposable credit card numbers, those are safer
4. Change your debit card PIN once in a while (don't get paranoid and change it after every transaction)
5. Keep the card company's phone number with you, block it if you get any suspicion (suspicious SMS etc.)
6. Keep your PC virus free
7. Use an updated browser
8. Don't perform the transaction if Internet connection is unstable
9. Don't get impatient, if the page says not to click back or refresh, do not (at least till 2 minutes. I have seen cases where the confirmation has arrived up to 3 minutes late)
10. Wherever possible, enable SMS alerts so that you can be notified about unauthorized transactions
11. Use Maxthon browser if your card company only supports Internet Explorer (it uses the same engine, but is still better than IE). Use Firefox if there is no restriction (avoid Opera, most payment gateways don't support it)
12. Look for the Verisign or similar company's symbol
13. Avoid public PCs (cyber cafe) like plague. If you must use those, use private browsing
14. Use virtual keyboard for typing password, it's available on every page these days
15. Last but not the least, check the website's reputation before performing the transaction
The above list may not be exhaustive, but will definitely make your transaction safer