How many of these traps have caught you?
having no conclusion and whimpering out on a weak "That's all" type of line
not timing the speech and running out of it before giving your prepared conclusion
ending with an apology undermining your credibility - "Sorry for going on so long. I know it can be a bit boring listening to someone like me."
adding new material just as you finish which confuses your audience. That belonged in the body of your speech.
making the ending too long in comparison to the rest of your speech
using a different style or tone that doesn't fit with what went before it
ending abruptly without preparing the audience for the conclusion. You need a transition or bridge to have them follow you comfortably.