1. Exploratory job search. I suggest that you conduct exploratory job search on a regular basis but no more frequently than once per year. You need it to know your market value and to assess the job market for your specific profession, skills, and experience. It encourages you to update your resume at least annually, even if you are not looking for a job. You also need it to get job search experience so that you do not get "stale". Job interviewing skills are like muscles. You need to train them to make them stronger. Just make sure to indicate confidential nature of your job search so that it would not hurt your current employment, and build strength to resist in case you receive an attractive offer.
2. Targeted job search. You conduct this job search when you are ready to change a job. You are still employed but you are determined to change your job, you career, or you just became aware that a massive layoff is coming up. In this case, you are well prepared for this experience by your exploratory job searches. Polish your interviewing skills, get feedback on your resume, target your employers of choice, and rehearse answers to most frequent 10 questions. Prepare your "elevator pitch" and get on the road.
3. Desperate job search. This is the timing for your job search that you want to avoid at all costs. You are not employed and are looking for a job. This raises a ton of questions: what happened? how long have you been unemployed? Your value as a professional is constantly being questioned. Interviewing experience becomes intimidating, and your chances for success diminish every day. Your responsibility is to avoid this type of search at all costs. However, sometimes you cannot. Unexpected layoffs happen and people lose their jobs. There are two simple tips I can give you in this case:
A. If you do not like your job but need one, try to stay there until you find a new one.
B. If you are laid off and have no job, engage in professional activities that show that you maintain professional value. Volunteer at a local User Group meeting, join a leadership network board, or organize a conference. Do freelancing on elance.com or guru.com. Or even open your own consulting company, depending on your line of work. Do not get depressed, do not become desperate, but even if you do feel desperate at some point, do not show it. Interviewers will sense it and if you are not confident in your strength, don't expect them to be. Stay confident and composed, and bad timing will no longer be a focus, your skills and knowledge will.
Remember: there is no bad timing, there is only lack of confidence.