When setting the parameter default value you can simply add a value into the Default Values - Non-queried expression box like this:
Now when you preview the report in the design environment this parameter will default to 1 and if you deploy this report to Report Manager and view it from there it will also default to 1.
The problem arises when you want to change it, if in the example you wanted to change the default value to 0 and you just replaced the 1 with a 0 then when you preview in the design environment the report will recognise this change and default to 0 but when you deploy the report it will still default to 1 (the value it was set to when it was first deployed)
This is easily fixed you just need to include and equals sign first.
Now when you re-deploy the report it will update the RDL on the Report Server.
For text (and dates, although using expression it better for dates) you just need to use speech marks e.g.
="Yes"
