Yes, you can start your preparation for IFS during B.Com, as earlier as you will start your preparation, it will be beneficial for you. Following are some tips to crack the examination:

You should learn about lot of things about society you are going to serve.
Follow standard books, stress on understanding things than memorizing them.
Go through previous year papers to check your knowledge and note the distribution of different topics.
Revision is one of the most important steps of the preparation process
Make concise notes and ready to revise material
Complete the preparation on time
Time Management-The Success Mantra
Cut off time from less important activities