How do you counter sentries and whips?

Frontiersman sentries and Kharaa whips are annoying. As a melee-ranged skulk, sentries target you instantly from across the room and nail you with precise and heavy damage. As a marine, whips are tough to take down and always hiding around corners.

How do you counter these immobile offensive structures effectively?

+1 to what Quinma said, but I'd like to add:

Sentries are actually not terribly effective against skulks. The reason is that sentries only attack in front of them, so if you can just jump over them, they can't hit you. Additionally, sentries require a nearby sentry-battery to operate. If no sentries are facing the battery (which is often the case, since they can't shoot through it), then you can take them all out at once by simply jumping over or around them and attacking the battery.

I find that sentries work better at gaining time: if a sentry is aimed at the power node, then the melee-enemies (which most of the aliens are) will need to take the sentry out before they can attack the power-node. This gives the marines time to make it to the room to protect it.

Also, if you're a marine and you're being attacked, running in front of a sentry could save your life.

Whips also are usually pretty easy to take out, due to their limited range. The only time they really cause problems is when they're placed around a corner - in that case, either try to run past or take a different route, to get a better vantage on them.

With a skulk vs sentry your only option is to avoid it or try to find a blind spot so you can get around back of it or past it. Commanders are often noobs and leave blind spots with sentry's.

As for whips it is pretty much the same thing. Avoid them until you have a path that gives you a decent enough angle to shoot at it and not get attacked.

You really have to remember where they are placed and plan your attacks based off that. If path A is blocked by a well placed sentry then play defensive there or go attack using path B.

Otherwise you wait for upgrades that will give you more options for taking down defensive buildings.