"Advice to young mathematicians" I have been suggested to read the Advice to a Young Mathematician section  of the Princeton Companion to Mathematics, the short paper Ten Lessons I wish I had been Taught by Gian-Carlo Rota, and the Career Advice section of Terence Tao's blog, and I am amazed by the intelligence of the pieces of advice given in these pages. 

Now, I ask to the many accomplished mathematicians who are active on this website if they would mind adding some of their own contributions to these already rich set of advice to novice mathematicians. 

I realize that this question may be seen as extremely opinion-based. However, I  hope that it will be well-received (and well-answered) because, as Timothy Gowers put it,


  "The most important thing that a young mathematician needs to learn is
  of course mathematics. However, it can also be very valuable to learn
  from the experiences of other mathematicians. The ﬁve contributors to
  this article were asked to draw on their experiences of mathematical
  life and research, and to offer advice that they might have liked to
  receive when they were just setting out on their careers."
My advice would be:
$\bullet $ Do many calculations
$\bullet \bullet$ Ask yourself  concrete questions whose answer is a number.
 $\bullet \bullet \bullet$ Learn a reasonable number of formulas by heart. (Yes, I know this is not fashionable advice!)
$\bullet \bullet \bullet \bullet$  Beware the illusion that nice general theorems are the ultimate goal in your subject.      

I have answered many  questions tagged algebraic geometry on this site and I was struck by the contrast between the excellent quality of the beginners in that field and the nature of their questions: they  would know and really understand abstract results (like, say, the equivalence between the category of commutative rings and that of affine schemes) but would have difficulties answering more down-to-earth questions like:    "how many lines cut   four skew lines in three-dimensional projective space ?" or "give an example of a curve of genus $17$".  

In summary  the point of view of  some quantum physicists toward the philosophy of their subject
Shut up and calculate !    contains more than a grain of truth for mathematicians too (although it could be formulated more gently...) 

Nota Bene
The  above exhortation is probably due to David Mermin, although it is generally misattributed to Richard Feynman. 

Edit
Since @Mark Fantini asks for more advice in his comment below, here are some more (maybe too personal!) thoughts:
$\bigstar$ Learn mathematics pen in hand but after that go for a stroll and think about what you have just learned. This  helps  classifying new material in  the brain, just as sleep is well known to do.
$\bigstar \bigstar$ Go to a tea-room with a mathematician friend and scribble mathematics for a few hours in a relaxed atmosphere.
I am very lucky to have had such a friend since he and I were beginners and we have been working together in public places ( also in our shared office, of course) ever since.
$\bigstar \bigstar \bigstar$ If you don't understand something, teach it!
I had wanted to learn scheme theory for quite a time but I backed down because I feared the subject.
One semester  I agreed to teach it to graduate students and since I had burned my vessels I really had to learn the subject in detail and invent simple examples to see what was going on.
 My students did not realize that I was only one or two courses ahead of them and  my teaching was maybe better in that  the material taught was as new and difficult for me as it was for them.
$\bigstar \bigstar \bigstar \bigstar$ Last not least: use this site!
Not everybody has a teaching position, but all of us can answer here.
I find using this site and MathOverflow  the most efficient way of learning or reviewing mathematics . The problems posed are often quite ingenious, incredibly varied and the best source for questions necessitating explicit calculations (see points $\bullet$ and $\bullet \bullet$ above).    

New Edit (December 9th)
Here are a few questions posted in the last 12 days which I find are in the spirit of what I recommend in my post: a), b), c), d), e), f), g), h).  

Newer Edit(December 17th)
Here is a fantastic question, brilliantly illustrating how to aggressively tackle mathematics, asked a few hours ago by  Clara: very concrete, low-tech and naïve  but quite disconcerting.
This question also seems to me absolutely original : I challenge everybody to find it in any book  or any on-line document !
If I could give a younger me advice, I would tell myself to lower my pride and accept as much help as I could in the times it was available to me, and shed my fear of humiliation and inferiority that kept me from even considering talking to others entirely let alone other mathematicians.

While you should strive to be more receptive to constructive criticism every day, I also think it's important to disregard criticism when it's source is from an individual that considers their authority in the field in question to be a conclusive presumption that must be held by yourself and all others.