When downloading a game from Steam, sometimes it downloads as fast as 1.8Mb/s but then it goes down to like 500Kb/s, how can I steady my connection?

More then likely you can not control it. There are a few reasons for this.

1) You can only go as fast as the fastest part of the internet you connect through. (number of hops and latency)

2) Your provider may have a lot to do with it because some are using "boost technology" and it messes up everything and really doesn't do anyone a favor.

Also what OS are you using?

It could be affected by the amount of users currently using your ISP's line at any given moment. That's why sometimes late at night I get faster speeds then during the dinner-time!