This is very useful piece to book Tatkal Tickets online from a blog Also, one of the comment added that erail.in is faster site than irctc.
1. In tatkal tickets even though the ticket is always booked from starting station of train to the ending station, there are different quotas for different source, destination pairs, which is extremely surprising. So for example today when I looked for Tatkal ticket from New Delhi to Nagpur on Gondwana express it was showing waiting. But for the same train when I looked for New Delhi to Bilaspur (which is the end station), it was showing around 86 tickets available. So I happily booked the ticket from New Delhi to Bilaspur, ofcourse after a few retries. So remember, always look for availability and book ticket from starting to ending station of train and anyways you are going to pay for the whole ticket in tatkal no matter what your source and destination are.
2. Tatkal days can be booked 5 days in advance. But a large number of times when booking a ticket from Nagpur to Delhi, exactly 5 days in advance and at sharp 8 am in the morning (which is when tatkal bookings open), I would find no availability. This puzzled me a lot and then I had one of those life changing moments, an epiphany, an aha insight. The trains didnt actually start at nagpur and in many cases the starting day would be one before the day they reached nagpur. For example Tamil Nadu express reaches Nagpur on 10th of June but it started from Chennai on 9th of June. So its tatkal booking would have opened on 4th of June (not on 5th) and would have got over that day itself. So lookout for the starting day of your train and book 5 days in advance from then. Also if I would have given my source station as Nagpur it will not have allowed me to book on 4th saying that "tatkal tickets can onlly be booked 5 days in advance". So again you would have to give source station as the starting station of train and then in boarding point you can put your actual station.
3. There is an option of "Quick Book" in the menu just below Plan Your Travel. This is something I discovered just a few days back and it is very useful. Basically here you need to know the train number that you would be traveling on and then in a single page you can fill all the details and get the booking instead of first looking at list of trains and then selecting one among them. Protocol I follow is that first at www.indianrail.gov.in I look at the availability and select a particular train I would like to travel on and then I directly go to Quick Book on irctc and fill in all the details. This really saves a lot of time, especially since indianrail.gov.in is not down that often as irctc. Also if a large number of people start using indianrail.gov.in for availability lookups than hopefully it should reduce the load on irctc and make it more responsive.