First of all - YES!
Write 5 at a time. New promos hit the "airwaves" straight away, so submitting your allotted 5 gets your forum name seen more often. It's a brief surge, they won't be traveling around for the next ten days together. If your forum is Classic you can use the Promote URL in your controls to promote your entire forum! Just use the Start page link and invite in the world!
Proof Read - Use Review Promotions in your controls to check the promos you just wrote for errors or confusing lingo. If you do find one just use the Delete option there (2 step process) and rewrite it.
One promotion per discussion/topic - not just a tip, this is a rule Delphi does enforce. By topic they do mean specif topic. If all your 5 promos lead to different discussions on insects for example, promote the unique subtopic of each of those discussions.
Don't recycle - While you may promote a thread over and over, keep the promos unique and fresh. Seeing the same promo over and over, may keep members away (ack!)
Promote off Delphi - Make sure your guest access is on, you can link your forum to your Facebook and Twitter pages when you submit the promos or anytime after! You can also use active content to place your forum promos on your off Delphi wegpages.