The trumpet reed, for that is what the lips really are, seems to operate very differently. In the case of the trumpet, the "reed" seems to operate in just the wrong way. When the pressure difference between the inside of the mouth and the inside of the instrument gets large, the lips are forced apart, not together. It would thus seem that it is impossible to operate the lips like the clarinet reed operates. One never gets to the point where the pressure difference is sufficiently large to close the reed.
However it is here that the other aspect of the lips comes into play. Lips are a much heavier reed than is that little piece of cane which is the clarinet reed. Furthermore, the lips are not very stiff. By tightening the lips one can alter the stiffness, but one can never make them as stiff as a cane reed. This means that the lips operate very differently from the cane reed. Where the cane reed responds almost instantly to the pressure difference between the outside and inside of the reed, the lips do not. They take a while to move. In particular it is very hard to move them on a time scale shorter than the natural resonant frequency of the lips.
So let us say that we tune our lips so that their resonant frequency lies just below one of the modes of the air inside a trumpet, and let us look at the interaction between the lips and the pressure oscillations at the mouthpiece of that mode.
When the pressure inside the instrument is high, this exerts a force on the lips tending to push them closed (since the lips bulge into the mouthpiece). However the lips do not close immediately, but rather just start to move under the action of this pressure. By the time they have closed and the air flow through them is at a minimum, the pressure in the oscillation of the mode has also dropped to a low value. Ie, the minimum air flow occurs when the pressure inside is a minimum. Similarly this lower pressure inside the instrument now creates a large pressure difference across the lips, causing them to start opening, but they actually open to their maximum when the pressure inside the instrument has gone high again, which makes the maximum air flow in to occur when the pressure inside is high.
What the trumpet is doing is using the time delay in the response of the "reed" (the lips) to convert what one might at first expect to be just standard resistance to negative resistance.
One of the key ways the trumpeter has of changing the pitch played is by playing the different modes of the air vibration inside the trumpet. (on a bugle, or hunting horn this is the only way of changing the pitch. On a trumpet or French Horn, there are also valves which open up loops of pipe to increase the length of the air tubing, changing the mode frequencies in that way. On a trombone, pieces of pipe slide within each other to change the length of the tubing.) What this means is that the natural lip vibration frequency is changed by tightening the lips (spreading out the lips so that less lip mass is inside the mouthpiece, and increasing the tension on the lips). If this is tuned to lie just below the frequency of the mode you want to play, then the lower modes are not amplified ( the lips respond too fast to the change in pressure inside and air flow in occurs when the pressure inside is still low, causing damping), and do not play. The mode just above the lip natural frequency responds most, since its pressure variations cause the greatest changes in the opening of the lips. (That mode is closest to the resonant frequency of the lips and thus the lips respond most to the pressure variations of that mode).
Really good trumpeters can change their lip tension so that they can play from the second mode up to 9th or 10th mode of vibration of air inside the trumpet. This is a change in frequency of about 5, which indicates how large a change in lip tension and mass the player must be able to produce. Bruising of the lips, or growth of callouses on the lips can be a disaster since this reduces the ability to change the mass and tension in the lips.