London's Heathrow (LHR) airport is a major European hub serving over 80 airlines to to over 180 destinations.   It is a massive airport spread over 4.75 square miles with 4 passenger terminals plus 1 cargo terminal.  To transfer between passenger terminals, it is required that travelers take a bus and sometimes a train.  Even if arriving and departing from the same terminal, passengers need to go through passport and security checks.  Every transfer passenger goes through a lengthy and meticulous passport and security screening process.  Expect lines and waiting time. Here are The Traveling Professor's tips for dealing with Heathrow Airport:

Avoid LHR:  Frankly, this is the best advice The Professor can give.  There is no other airport I can think of (maybe Madrid-Barajas Adolfo Suárez Airport comes close) that involves so much hassle in transferring through for connecting flights.  However, sometimes it is just impossible to get where you want to go without going through LHR.

Budget at Least 2 Hours:  Give yourself at least two hours, especially if changing terminals.  Expect gate changes too.  On our last flight we were scheduled to arrive at Terminal 5 and depart from Terminal 5, but after arriving at Terminal 5, our departure was changed to Terminal 3.  We needed to go through a quick security scan before getting on the bus to Terminal 3, go through passport control in Terminal 3, then full security scanning in Terminal 3.  Some passengers were required to go through another short security procedure before getting on the plane at the gate. By the way, expect to do a lot of walking to get to your gate. 

Pack Larger Bags Carefully:  While going through security, if a bag is flagged for one item regardless of how minor it is, the passenger will probably be asked to unpack the entire bag and have it thoroughly and painstakingly checked by security personnel.  Then the passenger will need to re-pack the bag.  Begging with security personnel that you will miss a connecting flight will not help matters.  So, The Professor's advice is to leave ANYTHING out of your large bag (electronics, liquids, coins, metals, soap, foods, creams) that could POSSIBLY cause them to flag and inspect your large bag.

Pack Smaller Bags Carefully:  When The Professor says this, he means it.  Put all liquids in smaller regulation bags and remove for security scanning.  Put coins, electronics (even charging cords), food liquids, anything with metal, phones, soaps, in plastic bags and remove them from your bag as it goes through security.  For instance, I carry an Eddie Bauer backpack when I travel.   I will put all phones, electronics, cables, coins, pens, charging cords, my small scissors, USB memory sticks, etc. in a large plastic baggie.  I easily remove this baggie from my backpack and have it screened separately.  If I left it in my backpack, and it were to be flagged, security probably ask me to empty and inspect my ENTIRE backpack, taking up valuable time. However, since it is separate from my backpack, they only need to inspect that individual plastic baggie, if it were to be flagged.  Of course, remove laptop computers, Kindles, etc. while going through security.

Passport Control: Bring a pen. Travelers may be asked to fill out an immigration form.  Ask to get this form on your flight.  Fill it in beforehand.  Have passports and other documents ready to go.  Upon arrival at LHR, look for and get a "Fast Pass" from airport or airline personnel that will get you on shorter lines. There are fast track services available at additional costs and by application, but it is only recommended for frequent travelers through LHR.

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