You may want to use a contact's first name to give your email a more personal touch.
While the merge tag used in the body of an email would be merge tag then comma, %%FirstName_%%, (read as 'Jane,') the merge tag used in the subject line has slightly different formatting depending on the table the information is getting pulled from.
Pulling from the basic members table:
%%FirstName_%%, Enjoy this free gift!
In a campaign, click on Add Merge, then click on the drop down arrow to change the Contact Field. Add Comma after the basic merge tag.
Alternatively, you can also use this method:
%%format "%s," [ merge FirstName_ ]%% Enjoy this free gift
(read as Jane, Enjoy this free gift)
*note the space between the open bracket and merge and between the underscore and end bracket
Pulling from a custom table:
%%format "%s," [merge Custom_account.tablename.mergetagFirstName]%% Enjoy this free gift
(read as Jane, Enjoy this free gift)
*note there is no space between the merge and open bracket and after the merge tag and end bracket along with no underscore