I am guilty of not practicing for a few months. Now I have started again I find that my notes are wobbly, or tremolo. I cannot produce a continuous breath, it goes strong-weak-strong-weak. What can I do about it ?

Long Notes...Long Notes...Long Notes... Just keep practicing. Just like when you started, your support needed to be built up. Also, do two or three shorter practice sessions.

As Taras12 says. Long tones.

Long tones build your embouchure strength.
Start with playing all the notes in your range for 10 seconds each day for one week.
Then play for 15 seconds each note, each day for one week. Then move to 20 seconds. etc.

Be consistent and move up slowly.

Ok, one thing mistaken is that long notes won't be the most efficient way to help your lungs. Best advice, do breathing exercises before you practice. Here's how:

Step 1: put your hand on your stomach, to make sure you are breathing the proper way.
Step 2: sit in a proper posture (DON'T SLOUCH!)
Step 3: breathe in with your mouth formed as if you are saying 'how', for four beats. Be sure to feel your stomach move out, indicating the proper breathing form.
Step 3: breathe out, as if saying two for four beats, steady and WITHOUT INSTRUMENT.
*Repeat with four beats in, and then eight out, four in, twelve out, four in...etc. *
Step 4: try the same technique with your clarinet, but without the hand on your stomach.

The reason I said to make sure you are breathing the proper way is because the muscle right below our lungs, called the diaphragm, will push down, filling your stomach with air and your upper body, therefore adding more air to use. This will be great for rebuilding your lung strength. Long tones will help, use the breathing technique above, and play a low E for four beats at 60 quarter note speed on metronome, then jump to the B natural (By pressing the register key) do leaps like this to build tone and lung strength. Hope this helped! Happy breathing!

Mad_fales