One thing to note is that you'll never get the top speed advertised for any connection due to transmission overhead; even so, you should be able to get close (within about 10-20%). Also, ISPs oversell their bandwidth, so if you run your speed tests when other customers are using their connection, you will notice the performance hit.
asks: "I'm infuriated every time I see that companies are raising their speeds when they can't maintain their current speeds. Here's my biggest issue: my grandmother signed up for the 3Mbps DSL plan through Verizon, however a speed test said she was only getting 750Kbps. Why pay for the extra bandwidth when she's not getting it? She downgraded to the 768K plan expecting to still have 750K. Wrong, instead her speed dropped to 300K. So, how about instead of companies constantly claiming to increase their speeds, they get their actual speeds correct. Comcast has done the same thing, I had their 6Mbps plan at one point, I got 2.5Mbps usually and sometimes 3Mbps, so they're all doing the same thing. In closing, with all these speed increases, why is my Internet not getting faster?"
What practices and tools do you use to test your bandwidth speed and how have you approached your ISP when the performance repeatedly fell short of your expectations?