I had written my SCJP1.4 in 2007 secured 91%.
I read Khalid Mughal for SCJP1.4. Prepared for 3 months worked on hell lot of programs. If you are in a position to answer most of the questions on forums like javaranch, it gives you a good feel on your preparedness.
I had written SCWCD5 on june 2010 and secured cent percent.
I read Head first servlets and JSP - twice and worked on all the servlets and jsp programs especially creating new tags and stuff which we do not work on regularly. Prepared for 2 months with an average of 1 hr per day - self study + had good project work experience on J2EE.
My idea was to devote some time for study and get a greater picture - something that you do not do on your day to day project schedule. Certification was just a test of my understanding.
For both the exams, I just worked on some mock exams only a day before the actual exam. Just to understand the format and get an idea of my subject grasp. Also, I used to buy my voucher for the exam on the day of the exam itself - just be sure that I'm really prepared.