I have admitted on this blog that writing the beginning of a story is difficult. You need to catch the reader's attention in a way that has not been tried before, which becomes more impossible each year and with every newly-published book.
Writing the middle of the story is even worse, in a way. By this point, the reader has committed to finishing your book, but a truly boring, bad, or unconvincing middle can still make them regret buying your book...and lose you their future purchases of your second, third, and umpteenth novel.
Recently, when talking with some of my friends who also write, I complained that every decent story follows similar guidelines. Something happens to the main character (who we're supposed to admire), the main character reacts, and then plot ensues. That is what you need to accomplish convincingly in the middle of your story - go makes some plot points happen!
However, as simplistic and general as that sounds, it is not helpful to a writer in the midst of a story. You have a main character, a foil or villain, friends and henchmen, a breathtaking setting, and enough secondary characters to make casting directors cringe at the prospect of your book one day becoming a film...but now you need the plot, encounters, adventures, setbacks, and other events that build tension and camaraderie for your audience. They should be cheering for your main character to win, and throwing taunts and tomatoes at the villain of your story. There are 3 ways to garner such involvement from your readers:
1. The world-changing approach -- Make every way station, pit stop, or bus depot vitally dangerous and important to your main character. For example, if your story takes place on the London underground, then every time the train doors open, your readers ought to wonder - and maybe fear - what will enter next. Say that your main character is a thief. Maybe she boarded the train to make her escape, but at each station, different people board and accost her. One newcomer could be a rival thief; the next would be her ill-gotten property's actual owner, and the third a police officer, called to quiet the incessant fights occurring in the thief's car.
The downside to this style is that it could exhaust your reader. The style of writing is evident in those novels that you finish in one sitting, despite their length.
2. The comedy of errors approach -- Keep all events light, humorous if possible. In keeping with the London underground example, your heroine would still be accosted, but not at every turn. Maybe in trying to avoid her thieving rival, she stumbles across an unrelated problem that only she can solve. (I.e. A man has just dropped an engagement ring down a grate. The thief/hero retrieves it for him.) The other encounters could be minimized also, in that the stolen property's original owner might not know that your main character is the thief, and the police officer could be interrupted by a more serious crime, and leave without arresting her.
The problem with this approach is that, though it is less intense and easier for the reader to handle, your audience may wonder why they're reading it at all.
3. The balanced approach -- Mix the serious encounters with the lighter ones, alternating randomly. Have the rival thief steal the hapless man's engagement ring. Then, while the hero/thief is stealing it back, the owner shows up with a police officer. The hero/thief grabs the engagement ring from her rival before he runs away from the authorities. When the police officer searches the hero/thief, though, only the engagement ring turns up, because her rival lifted the object that she stole earlier!
This mixture of intense and funny moments will reveal more facets of your characters. (For example: How does the hero/thief react when facing her rival as opposed to when she is faced with a blunt accusation from her most recent victim?)
Mixing the content of your middle story also gives the reader a chance to properly enjoy the characters, without either feeling rushed from scene to scene by the urgent pace and intensity of the plot, or feeling bored at the consistently whimsical and inconsequential events. Pick the style that works best for your story.