One thing to be careful of is over-inflated plan specifications, i.e. overselling.

Chang you company and you host plan ..
GOOD LUCK

Do you checked your access log with an analyzer like Webanalyzer what is causing the most traffic ?
Also, do you send a lot of mails, big attachments that are loaded often - all this may be a problem, each one maybe just small, just count them together.
Maybe doing this before switching the host

Thanks for all the responses thus far, people. Much appreciated.

I am now looking to switch hosts once again, but ensuring this time that a prospective host could handle traffic of around the amount that we experienced last month (which was 140,367 MB).

Could any of you offer any sound advice on this matter? I am reluctant to make the same mistake as before, moving to a new host only to be charged excessively anyway!

That traffic seems rather excessive... how many active/online users do you have at once?

You might try a step up from shared hosting maybe a VPS or entry level dedicated. They usually come with more than enough bandwidth to cover your 140GB a month.

Looks like a very active forum you have just a tad over 2,000 members with over 40,000 posts. this indicates that your members are rather active. I have an extremely active forum only 4 months old but we've just hit 104X members approaching 40,000 posts. We are doing 2.6Gigs per day (on a good day) with an average of 1.9Gig per day!

So it is very normal because if you look at it from a page views perspective we are getting like 50,000 page views each month and climbing (that is a lot of data being fetched from the server!)

Congratulations on a successful and I assume active forum from what you've stated.

Check through your databases, someone might of hacked your Server, and they could be using your host, under your name. But without you knowing.