Show that $a_n$ is decreasing 
$a_1 = 2, a_{n+1} = \frac{1}{3 - a_n}$ for $n \ge 2$. Show $a_n$ is decreasing.

First we need to show $a_n > 0$ for all $n$.
$a_2 = 1/2$ and $a_3 = 2/5$ and $a_4 = 5/13$ 
One way we can do this is by showing $3- a_n > 0$. Thus suppose it holds for $n$ then we need to show $\frac{3(3 - a_n) - 1}{3 - a_n} = \frac{8 - 3a_{n}}{3 - a_n} > 0$, which means showing $8 > 3a_{n}$, but I'm having trouble showing it.

Let us go for the overkill, i.e. to find an explicit formula for $a_n$. We may set $a_n=\frac{p_n}{q_n}$, with $p_1=2,q_1=1$, then deduce from $a_{n+1}=\frac{1}{3-a_n}$ the recurrence relation:
$$ w_{n+1}=\begin{pmatrix}p_{n+1} \\ q_{n+1} \end{pmatrix} = \begin{pmatrix}0 & 1 \\ -1 & 3 \end{pmatrix} \begin{pmatrix}p_{n} \\ q_{n} \end{pmatrix}=M w_m\tag{1}$$
The characteristic polynomial of the matrix $M$ is $x^2-3x+1$, having roots $\frac{3\pm\sqrt{5}}{2}=\left(\frac{1\pm\sqrt{5}}{2}\right)^2$.
By the Cayley-Hamilton theorem it follows that $x^2-3x+1$ is also the characteristic polynomial of the sequences $\{p_n\}_{n\geq 1}$ and $\{q_n\}_{n\geq 1}$, hence:
$$ p_n = p_{+}\left(\frac{1+\sqrt{5}}{2}\right)^{2n}+p_{-}\left(\frac{1-\sqrt{5}}{2}\right)^{2n}\\ q_n = q_{+}\left(\frac{1+\sqrt{5}}{2}\right)^{2n}+q_{-}\left(\frac{1-\sqrt{5}}{2}\right)^{2n}\tag{2} $$
and both $p_n$ and $q_n$ are linear combinations of the Fibonacci and Lucas numbers $F_{2n}$ and $L_{2n}$.
Interpolating through the values of $p_0,p_1,q_0,q_1$ we get:
$$\boxed{ a_n = \color{red}{\frac{F_{2n-5}}{F_{2n-3}}}}\tag{3} $$
and $\{a_n\}_{n\geq 1}$ is a decreasing sequence since $\{F_n\}_{n\geq 2}$ is an increasing sequence. Moreover, from $(3)$ it follows that:
$$ \lim_{n\to +\infty}a_n = \frac{1}{\varphi^2} = \frac{3-\sqrt{5}}{2} \tag{4} $$
as expected, since if $\{a_n\}_{n\geq 1}$ is converging, its limit has to fulfill $L=\frac{1}{3-L}$, i.e. has to be a root of the previous characteristic polynomial.
