Okay, so a lot of people have suggested various books, but how about some actual tips?
Check the rate on every ECG using the 300 150 100 75 60 50 rule.
Determine if the rhythm is normal- that is, is there a p wave before every q wave and a q wave after every p wave? Both conditions are necessary. Is the person in sinus arrhythmia, in which everything is normal except the change in the pulse with breathing?
Look at the PR interval. If it's too long, for example, the person could be in first degree heart block.
The voltages on leads I and AvF should be positive in people older than infants. If BOTH of them are negative, suspect that someone has reversed the leads when doing the ECG.
High voltages in lead V1 means right ventricular hypertrophy. Maybe the person has pulmonary stenosis. High voltages in lead V6 means left ventricular hypertrophy.
These are just some basics but it will get you started looking at ECGs. Does anyone have anything else to add? I think the most important thing is to go through ECGs systematically.