Theres probably a whole bunch of things I could suggest but here is what I can remember at this point in time!
When planning for Europe we thought we could get away with 20-30 euro a day (about $60 NZ) - this is not going to happen. I would say the minimum to budget would be 50 euro - on the odd day you will only spend 20 euro but then next you may spend 80-100 so 50 is a good balance to aim for.
Extra costs we didn't think about:
Washing machines and dryers - these can cost up to 7 euro a wash (yes, $14) and the dryer can cost the same so its a good idea to team up with someone and do a wash together. Your clothes will shrink and get destroyed on this trip (we'll blame it on the tumble dryers - not the amount of food and drink were consuming!) so don't take your best clothes!
Bottled water - this can vary from .30 euro to 4 euro a bottle, depending on where you are and when its hot - and it gets very hot - you are drinking up to 4 small bottles a day. You can save by getting the big bottles but then you are lugging it around.
Public toilets - Oh Europe and your public toilets!! I swear some countries don't even have them, Paris was the worst, I remember seeing one on the whole day of walking! You do have to pay for them in other countries but its a maximum of .50 euro so its not that bad.
Snacks at gas stations - if you do a bus tour you can stop at a gas station for toilet breaks / food stops up to 3 times a day and snacks all add up. Its a good idea to stock up on food at the supermarket when you can - just don't eat it all in one go!
Optionals - Beware the optionals on bus tours!! Most tour guides get a commission from you eating a restaurant that they recommend - the deal is they bring all of their bus tours to that particular restaurant then they get a percentage of each meal bought. We caught onto this pretty quickly and the food was not good at all. The most disappointing dupe of all was the 'amazing' 5 story night club in Prague - again we pay, tour guide gets a cut. Imagine a huge public toilet block, put a dance floor in the middle of it and disco smoke and thats the 5 story club in Prague. The best example of the optionals occurred in the Swiss Alps though. We stayed in a small town with not a lot to do (so we were told) and the options we were given were sky diving which was hundreds of dollars or a 2 hour train ride up a mountain to see snow and views of the mountains which was the equivalent of $170. We didn't want to pay that much so went for a wonder around town. What did we find? An 8 frank gondola ride ($11) which took us up a mountain where we went on a stunning hour and a half walk seeing the most spectacular views and had lunch at a village that was full of restaurants and cafes, then took the 8 frank gondola ride back down. Were we told of this? No. Why? Because the tour leader would have made no profit. Moral of this story is DO YOUR RESEARCH. Just because the tour guide recommends it it does not mean that that is the only option or that you'll be missing out on an amazing experience - cause chances are you'll find a better one just around the corner at a fraction of the price.
Transport on free days - every second day you will be off the bus so you have to find your own transport. You can get a tube day pass for around 6 euro a day or catch the bus which will be cheaper. If your exploring a smaller city its always best to walk cause you get to see more and save your cash.
Smoking - Having gotten used to the no smoking law in New Zealand restaurants and bars I forgot that this didn't apply to the rest of the world and everyone seems to smoke in Europe! So when you find a restaurant make sure you check it out first before you sit down to eat - theres nothing worse than eating your food when someone on the table next to you is blowing smoke your way!
What I found handy that I packed:
Micro towels - debatable - they are quite small so they don't cover much! And they didn't dry as quick as I thought they would but it was still probably faster than a normal towel.
Take mirror if your a woman - Imagine the following scenario - you have to up, showered, dressed, packed, have eaten breakfast and sitting on the bus by 7am. There are four girls in your room - and one bathroom (insert shock / horror face here) so I recommend that you take a small mirror so you can either do your make up on your bed or the bus.
Kathmandu toiletry bag - I love my toiletry bag! Its sizable enough to fit everything you need in it but small enough to pack away nicely - it also has a handle on the inside so that you can hang it up in the shower - genius!
Walking shoes - this is the most important in my opinion. You are doing so much walking when your traveling so its a necessity to invest in a good pair of walking shoes. I bough a pair of Nike Airs and it was the best thing I bought for the trip - I have literally worn them everyday for the last 3 months! Wait till you get to England though cause there cheaper here! Also try and get a good pair of sandalls cause we got really bad blisters from wearing ours around!
Flask - I didn't need my flask in the end. I thought that I would make a flask of coffee or tea in the morning and take it with me on the bus. Reality is the first toilet break isn't for 2-3 hours so drinking a flask of tea is not ideal unless you have a bladder of steal!
Food intolerance's - I was surprised by the amount of gluten free food I found in Europe and Top Deck provided gluten free lunches and dinners when they were providing a meal for us (you get about 6-8 Top Deck made dinner and lunches on the trip) and it was the best food - much nicer than most of the restaurants that we visited. The only struggle was breakfast so its good to have some rice cakes on hand if there is no yoghurt or fruit available. Top Deck will re-enburse you if you keep the receipt for foods that you buy to do with allergies so that was also a bonus. Apparently its harder if your lactose in tolerant but its advisable to call whoever you are traveling with before you leave incase you have to stock up before hand.
Spare tooth brush - Take a spare tooth brush cause when you drop yours on a nasty as hostel floor and the only option is to get one at the next supermarket for $10 you will wish that you had brought a spare!
Student I.D discounts - If you have one, take it. You can get into places like the Vatican City for up to half price so you save a fair bit!
Medications - Make sure you pack pain killers and hay fever tablets if you get hay fever. In NZ spring time is September so I didn't even think that Spring time in Europe was April / May and I actually had the worst hay fever of my life. Luckily I had thought ahead and got heaps from the doctor but it was really to last me months so I didn't have to go to a doctor in England for it but I ended up using most of it in the first month I was away. Also remember that not only do they speak a different language in Europe - everything is written in a different language. So if you take specific medications get in before you go so you can actually read the packet!
Hope these tips help :)