Q:

How can I run a profitable garage sale this summer? 

Have you done garage sales before?   Any key learnings to share?   How to do one well, what to do, what not to do? 

A:

Advertise in advance!   If people do not know it's going to happen, they can't show up.   Common ways are: 

Signs in the neighborhood.   If you live in a development/subdivision see if the HOA will let you put up a sign at the entrance. 
Small ad in the newspaper. 
Ads online on sites like Craigslist.   This often has the benefit of being free. 

A:

We don't do garage sales ourselves anymore because we found we did not make much money for the amount of time and effort we put into them.   
What we are doing instead is we have sold items through free advertisements on usedcanada.com.   Our success rate there has been excellent.   We have also donated household furniture and goods to charity and received a charitable tax donation receipt (and they even brought a truck and picked up the donated materials themselves). 
I have to admit though, that we go to garage sales ourselves -- mostly just because they are fun.