I have been traveling since 8 and at 18 I started my first "unsupervised" travels. Since then, I went on 2 Europe Backpacking trips (including my current one), countless of short trips to Malaysia & Thailand and trips to China & Japan. I don't claim to be even close to being experienced but I hope these tips I have picked up along the way will be useful. Here goes:
1) Mindset: This is the most important advice that I can give to any inexperienced travelers. Always assume that you will get the chance to re-visit the place - your first is not definitely your last. Many people tend to treat their first visit to any cities as their final visit there and will squeeze in as many things to do or see in their itinerary. You get things like attempting to "cover" Paris's Louvre, Eiffel, Champs-Elysees, Notre Dame, etc... in just 3 DAYS. Rushing 3-4 attractions a day gives you hardly enough time to fully enjoy any place. The rush makes fatigue set in earlier, making future destinations not as pleasurable. Worse of all, what most don't realize is that in the midst of all hustling and bustling, you miss out on the most important experience - the "local" experience, where you understand, learn and live about the true culture of the place.
Note: Assuming that you will get a chance to re-visit doesn't equate that you definitely will. Let the place decide for you. Make the decision after leaving the place and not before. In this way, you will not feel hard-done if you end up missing out on certain sites, unable to attend certain events or can't try certain local food. Furthermore, with new innovative ways to make traveling so much more convenient and cheaper, there shouldn't be a reason why you can't re-visit certain places again. So take your time, fully enjoy the experiences and bask in the true reflection of the city.
2) Set your objectives before the trip & adjust the attitude accordingly: What do you want to achieve from this trip? Do you want to see as many sites as possible, meet new people or party all night? Experience local cuisine, live like a local or simply to unwind? I could come out with more possible objectives but ultimately you got to ask yourself why are you on this trip. From there, set your expectations and adjust the pace, look for places or experiences aimed to meet those expectations, tailoring your plans accordingly. Many travelers just dive in without even giving a thought about the objectives and come out disappointed ("Everything looks the same after awhile" or "I don't even know what I'm looking at after awhile"). You obviously can't achieve anything without even setting what you want to achieve. On top of that, travelers in a group tend to get along better if their objectives are similar.
3) Expect Mistakes: In any trip, no matter how meticulous the planning was, hiccups will happen. It can be bad weather, inaccurate maps or directions, strikes, unexpected closure, etc. EXPECT THEM! Be flexible and be ready to plan on the go. One wrong turn doesn't mean the destruction of the entire trip. Remember Tip #1: there is always a chance to come back if you choose to after the trip.
4) When travelling in a group, get everyone involved: Besides allowing everyone to sync their expectations, being involved in the planning simply makes the entire trip more rewarding and enjoyable. Be involved, know what is happening and have a stake in the ultimate outcome of the trip. Even if its a small part, at least you know you played your part and can enjoy the fruits of your labour when the time comes. For the leader of the group, no matter how minor a part is (even like in-charge of coming out with cooking/cleaning schedules), make sure everyone gets one.
5) Open your mouth and ASK: This saves time and reduces unnecessary errors. Enough said.
6) Be sociable but not imposing: This is especially for travelers staying in hostel or couch-surfing. Don't let the usual stereotypes stop you from socializing but at the same time do not impose yourself on others. Know when to stop the conversation. Gauge the situation before asking personnel questions. Do not make the people around you uncomfortable. The worst thing is to have to come back to a group of uneasy house/hostel-mate.
7) Don't be lazy, do your research: This goes in line with Tip #1. How do you know which are the sights and experiences that can truly reflect the spirit of the city? What to eat and where to party to be able to sample the local culture? All these have one simple answer - R-E-S-E-A-R-C-H. There are enough guidebooks (to buy or borrow), blogs, websites and forums for you to make an informed decision. Even asking locals online is considered good research. Research can save you time and money too. Of course, I don't mean doing detailed research that boils down to every single minute of your time. General research will suffice, allowing you to have an idea of what to see and do given the time you have.
That is all for now. I will go into the details about each advice as time goes by, giving a clearer idea on how each piece of advice will fit into your travels. Feel free to comment or contact me (via email or twit) if you have more to discuss regarding this. I love to discuss travel issues. Stay tuned for my Swiss Experience.