‘I am collecting these tips from other moderators (Van) and myself.’

If you cannot reach 90% of your ISPs advertised speed you might have a problem.

When you need help please add these things into your post:
What OS (operating system) are you running?
What service do you have and what is the advertised speed for download/upload.
Which tweak programs have you already installed and run this test:
http://www.dslreports.com/tweaks (copy and paste the result (URL) address in your post)
What speed do you get? http://www.testmy.net/
Also download this Tracert and Ping program (made by PHP)
http://testmy.net/forum/index.php?action=dlattach;topic=3341.0;id=1352
(Click on autocheck.exe, when it’s done just right click and paste all the results in your post)
Never connect to the modem via USB! Always connect your modem via Ethernet (NIC) and make sure you have the latest drivers for your NIC.
If you have a router set your NIC to 100mb full duplex for both Cable and DSL.
If you don’t have a router set your NIC to 10mb half duplex for Cable, for DSL it depends on the ISP.
Make sure to have the latest firmware for your router.
Power-cycle your modem, unplug it for at least 30 seconds.
Scan and remove all spy-ware.
Spy-bot: http://beam.to/spybotsd
Ad-aware: http://www.lavasoft.com/support/download/
I would also turn off your software firewall when testing your speed, software firewalls might be slowing you down.
If you need software firewalls try these two:
http://www.agnitum.com/
http://www.sygate.com/
Scan and remove all viruses.
If you don’t have one, AVG is free:
http://www.sygate.com/
Make sure you have all of the latest Windows Updates (MAC – make sure to update also)
I would also run this program to clean your system:
http://www.ccleaner.com/

Van has the low-down on Cable-nut so follow him.
http://www.testmy.net/topic-1013

Windows XP/2000/ME/9x registry patches:
http://www.[this.site.has.been.blocked.for.spamming]/downloads.php
Windows XP SP2 patch:
http://www.[this.site.has.been.blocked.for.spamming]/read_articles.php?id=1497
If you have a MAC this might help:
http://www.versiontracker.com/dyn/moreinfo/macosx/12591

Reboot!