How can I prevent blown speakers?

My friend told me to turn the volume down then turn off the deck (not let he car auto on/off it)

and when I turn it on to gradualy raise the volume back up slowly instead of suddenly letting the volume reach that lvl.... anything more I can do? Break in period?

just make sure your amps aren't overpowering. Tunr the amp gain all the way down and the radio as loud as you will ever have it, then slowly turn up the gain till you hear distorsion form the speakers then back it off a hair. Then just donw turn on your radio louder than that point.

Overpowering a speaker rarely is the reason for it blowing. The actual reasons that I have most commonly seen are:
1. Clipping: As in when you turn your radio all the way up, crank every knob within reach to its max setting (ie. bass and gains), and still want more volume from your speakers. What you are actually doing is telling your poor source unit to provide more signal than it can possibly produce to your processors and amps downstream. This leads to it producing spikes of voltage, which cause your amp to shoot the speaker's cone outward, and just leave it hanging for a few milliseconds before it can return to its resting position. The amp should have a clean enough signal to control the amp in both directions, and do so with authority, which means you need power.

2. Improper crossover points, or total lack of crossover points: This sad occurrance has led to the death of many midbasses that are just slung into a door without any form of enclosure to provide them with needed suspension. The little 5.25's are being asked to play from 100Hz and down, and they just simply aren't designed to handle lowest frequencies of that range. So don't try to make speakers work miracles. Combine this with too much power, and you get amazing excursion from tiny midbasses, until one day when all of the noise just stops.

3. Underpowering: People love to bitch about overpowering speakers, but excess power will NOT blow a speaker. (Please don't post a reply saying it will, and some BS like "If you bridge a Power 1000 to a tweeter, it'll blow!!") I am not talking about extremes here, just normal amps and speakers. However, look what underpowering leads to. Let's say you have 2 12" subs that require about 250W for optimal performance in their enclosure. Now, say you are running them with 50W each, which happens every day, sadly. You wonder why your subs aren't rattling the trunk like you want them to, so you crank everything regarding volume and bass up until the knobs just stop twisting. Your source is clipping, you are creating a distorted trash signal, and your speakers will soon die. You would have been much happier with one of those 12" subs with the same amp bridged to it. It would have been louder than the underpowered pair, and sound much better!

Basically, these problems are actually a chain of events; one leads to another, and before you know it, speakers are blown. If you have a pristine signal, enough power, and the necessary filtering, your speakers should only blow due to old age or manufacturers' defects.