There is an excellent read about wa in
Making Sense of Japanese.
Many books or teachers will tell you wa は can be interpreted as "as for...". While this is the general rule of thumb you should exercise caution as it is not always correct.
To be safe you could use this explanation. Usually people expect some interesting information after wa whereas ga is mostly a subject marker.
Example#1
Basic declaration.
watashi ha Universal Studios Japan ni iku.
I am going to USJ.
*Japanese wouldn't use 'watashi wa' here. They would simple say, 'Universal Studios Japan ni iku.' I am using it to better illustrate my point.
Example#2
Q. Who is Canadian?
A. I am Canadian.
A. watashi ga Kanadajin desu.
If I said 'watashi wa kanadajin desu'. People would turn around and think I'm wacky because I would be declaring to everyone for no apparent reason, 'I am Canadian, hurray!' Or they may think I am emphasizing for no apparent reason, the fact that I'm Canadian and not some other nationality.
Instead I use ga to say in effect, 'I am the one who is Canadian' or '(I) am Canadian'.
I hope that is a good example. I'm not an expert so if someone has something else to say please share it.
She kisses me nicely.