First Publication as a PhD student I'm in the process of writing my first paper as a PhD student and was wondering if I could get some general advice regarding the following:


How many Journals to consider submitting a manuscript to and how much impact factor plays a role here. Some say aim high as possible.
How much time should I spend actually writing the paper? Do I continue working on other projects at the same time as writing the paper?
What other considerations should I keep in mind after submitting the manuscript? e.g. should I be writing up another manuscript for another journal-just in case?


I would be very grateful to hear about other people's experiences, etc with their first publication. 
Of course,I've read a lot of things on the internet and I've spoken to my supervisors too but I believe you can never learn too much. 
My project in the mathematical sciences with applications in biology/medicine.

Thanks.

EDIT: I found the following links very useful: one, two
A Professor here.  


In general, yes, aim as high as possible.  In practice, I do an educated guess as to which journal to send it to after seeing the final product/paper.  If the student is capable and motivated, then I would work with the student to get the paper into the best journal.
As much time as possible.  Papers only get better over time.   There are always avenues to improve it, technically or presentation wise.  This is where your supervisor's experience is critical.  Only he/she will be able to push your paper to the required standard and also steer you away from pitfalls.
Many directions here.  You could beef up the paper with new results, incorporate more assumptions or proof the general case :)  Alternatively, you can work on a new problem or a related problem. Problems tend to reveal themselves as you dig deeper.  All my students start on a new/related problem after they submit a paper.  They might choose to go on a holiday first though.


All the best ...