"methods for constructing a musical motive"
For me, it doesn't work like that. A melody comes to me, and in retrospect, I find the motives. I can't really explain how that works, a motive is just anything that I recognize as a distinct unit - some kind of pattern.
Whether something is a pattern is to some extent subjective. Melodic and rhytmic properties are to some extent independent: it may be a particular rhythm, or it may be a sequence of melodic intervals, or even a single interval. It might even not be pinned down to intervals, but just a particular pattern of melodic motion. Whether something is a motive or not is also context dependent - you might not recognize something as a pattern if it occurs only once; repetition, or even re-statement or a variation might trigger recognition on the listener's part.
This all sounds very abstract, but in practice it's usually not that hard to find motives.
Once you have motives, it really helps in creating larger forms.
You can use them to "fill in" parts, kinda like the way some people doodle figures in the margin of their notebooks. Of course you should use your taste and ears and let them do their work as well. Simply mechanically filling everything up with the same motives can become boring.
Or you try and feel whether the motives you found have a tendency to "go somewhere". For example, a rhytmic motive that starts with long notes, and gradually accelerates into shorter notes may be restated and varied to accelerate even more by adding even more shorter notes. Or a melodic motive that starts with step-wise motion and continues with ever larger leaps can be exaggerated by adding more, even larger leaps.
Sequences are also a very tried method to work out motives. Sequences take a motive (typically, consisting of both a rhytmic and a melodic pattern) and systematically repeat it at another pitch. This is done typically 3 times, and typically the change in interval is persistent for all legs of the sequence.
Because the sequence is both repetition as well as variation, this is a very powerful device to develop motives. You can play with the size and motion of the interval to invoke a sense of tension or relaxation. For instance, if you use an ascending interval of a second or a third typically feels like an increase in tension, and often this can be used to force a modulation to another key.
Here's a nice example of motives and sequenes: http://musescore.com/user/13172/scores/130814
(BWV 847 The Well-Tempered Clavier Part I Fuga II)
The visualization over here shows the sequences as a little stairs icon:
http://bach.nau.edu/clavier/nature/fugues/Fugue02.html
Check out the text in that page too. It does a great job of explaining how the motives in the theme are used throughout the piece.
