如何实现Android buttton按钮美化?请大家帮帮我
以下是源代码，那个button按钮怎么美化呢？，请大家帮帮我吧。
比方说如何让button按钮有按下的效果，如何让button按钮实现颜色的渐变。

Java code

package com.edu.ytu.cn;import android.app.Activity;import android.app.AlertDialog;import android.app.AlertDialog.Builder;import android.content.Intent;import android.os.Bundle;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.view.Window;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.EditText;import android.widget.ImageButton;import android.widget.TextView;import android.widget.Toast;public class MyLoginActivity extends Activity {    /** Called when the activity is first created. */    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        requestWindowFeature(Window.FEATURE_NO_TITLE);//设置窗口无标题        setContentView(R.layout.login);                final EditText username_et = (EditText) findViewById(R.id.username_edit);        final EditText password_et = (EditText) findViewById(R.id.password_edit);        final TextView regist_tv = (TextView) findViewById(R.id.register_link);                Button log_bt = (Button) findViewById(R.id.signin_button);                log_bt.setOnClickListener(new OnClickListener() {                        @Override            public void onClick(View v) {                                final String username_str = username_et.getText().toString();                final String password_str = password_et.getText().toString();                // TODO Auto-generated method stub                if(username_str.equals("admin") && password_str.equals("123")){                                        Intent intent = new Intent(MyLoginActivity.this, MenuActivity.class);                    startActivity(intent);                }                else{                    Toast.makeText(MyLoginActivity.this,                            "账号或密码错误，请重新输入！",Toast.LENGTH_SHORT).show();                }                            }        });                     regist_tv.setOnClickListener(new OnClickListener() {                        @Override            public void onClick(View arg0) {                                LayoutInflater inflater = getLayoutInflater();                View layout = inflater.inflate(R.layout.regist,                     (ViewGroup) findViewById(R.id.rg_dialog));                                 AlertDialog.Builder builder = new Builder(MyLoginActivity.this);                                 builder.setTitle("请认真填写您的信息")                                              .setView(layout)                                              .setPositiveButton("注册", null)                                              .setNegativeButton("取消", null).show();                            }        });    }}

大家一定要帮帮我呀，谢谢大家了。

[解决办法]
参考这里：
http://blog.csdn.net/aminfo/article/details/7782253
[解决办法]
为不同状态的ICON写布局 (默认/聚焦/按下)
[解决办法]
让UI做美丽些。