You've been training for a few months now. Your goals have been set (some of them may have even been achieved already) and you're happy with your routine thus far. You could be forgiven for thinking that it's all plain-sailing from here onwards. Unfortunately for you, it only gets more complicated. The onus at this point is to avoid a plateau, where you'll stop seeing and feeling progress and quite probably become demoralised and frustrated. When this occurs, the "F.I.T" principle should be applied in order to overload the muscles and surprise the body into instigating further growth and strength gains.

I understand that last paragraph was a little vague and disjointed. Please, bear with me. The "F.I.T" principle is something taught to most Physical Education students at a GCSE level, but is rarely conveyed as an invaluable weight training tool. As you've guessed, it's an anagram.

Frequency

Frequency is related either to the number of times you complete a specific exercise (a repetition) or the number of times per week that you complete your full exercise regime. I would advise against increasing the number of times you visit the gym, for safety purposes and because there is a risk of not recovering in time for subsequent sessions. However, an increase in repetitions is often the best medicine for a stale routine, and can improve muscle tone heavily.

Intensity
Intensity refers to the difficulty involved with performing an exercise. In terms of weight training, it refers to the amount of actual weight you're lifting. As I explained in Weight Training for Beginners, increased weight will result in increased strength and size gains, and is preferred by most gym users as the logical next step when a routine become redundant. For those users who aren't interested (for personal or sporting reasons) in increasing their weight or size, this is a less viable option.

Time
To make use of the "Time" principle (you guessed it already, I imagine) you should aim to work out for longer. There are two ways of doing this. Firstly, you can add sets of new exercises to your routine, thus stretching your workout past previous dimensions. You may also consider your lifting technique. If you start to perform longer, slower movements you can place the muscles under duress for extended periods and literally force them into growing. The time principle is obviously dependent on your prior commitments on a day-to-day basis.

Of course, the "F.I.T" principle is by no means exhaustive. There are many other ways to overload the muscles to break the "glass roof" most gym users encounter after an initial month (or two) of training. It is perfectly acceptable to completely revamp your workout plan and incorporate all-new exercises into your routine. However, if you haven't the time or inclination to do so, stay "FIT".