Hey, leeneia-
Sorry to hear about your woes. The church I used to attend, though smallish, had wonderful acoustics and we never ever needed reinforcement for the speaker or the musicians. But that aside...
There's lots of good technical advice upthread, and good sources. But what you may need more is the human factors stuff, so I'll toss out a few thoughts in no particular order.

If you can't talk 'em out of the PA, then...

(1) make a habit of referring to it as "sound reinforcement" rather than amplification, and you may achieve a subtle mind-shift in the desired direction.

(2) try to have one single person assigned to getting the sound right (hopefully a competent one with some common sense and not a rock roadie), and -- important -- try to arrange to FORBID anybody from helping. I'm only half kidding on this. Too many cooks...

(3) consider the differing auditory needs of various ages of folks in the congregation. The more, er, experienced folks may have high frequency loss which makes discerning the consonants in speech difficult. Amplifying the mids is exactly the wrong thing to do for these listeners, it makes it louder but much less intelligible.

(4) IMO you need TONS of headroom in a sound reinforcement application. You want gobs of fidelity and not so much sound pressure. If you have to have the master volume past around 3, you may need a bigger amp.

(5) Speaker clarity is a black art. All the headroom and EQ in the book won't fix a cheap, muddy speaker. I happen to like Peaveys and JBLs, don't think much of Behringers and absolutely despise Kustoms for this trait. Sorry, 4 & 5 were techie and not human factors. Back to topic:

(6) Final comment is on mic use. Lapel mics if they are good quality solve the problem of the speaker constantly getting too near and too far. Fixed lectern mics are too often of the vocalist type which have very touchy response -- okay when used by someone with the experience to subtly modulate their distance from the mic according to what they are putting out. Most people do exactly the wrong thing and swallow the mic when they want to get loud.

Hope this helps. I also hope you can hold the line on the prelude time. The congregation needs time and the appropriate environment to shift from 'getting there' to 'being there.' Make the sound checkers start earlier.
Cheers
-Glenn

BTW always make the sound checks in an empty room, but then pay attention to the result when the congregants fill it up -- it will be different. Make the appropriate compensation next time. -G